diff --git a/chapters/3.application-design/3.4.use-cases.tex b/chapters/3.application-design/3.4.use-cases.tex
index 6935128..824bb70 100644
--- a/chapters/3.application-design/3.4.use-cases.tex
+++ b/chapters/3.application-design/3.4.use-cases.tex
@@ -68,6 +68,13 @@
 {Σενάριο χρήσης 1 - Εναλλακτική ροή 3}
 {3-4-use-case-sign-up-alternate-flow-3}
 
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-sign-up-sequence-diagram}
+    \caption{Σενάριο χρήσης 1 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-sign-up-base-flow-sequence-diagram}
+\end{figure}
+
 % ===== =====
 % Use case 2
 % ===== =====
@@ -121,6 +128,20 @@
 {Σενάριο χρήσης 2 - Εναλλακτική ροή 2}
 {3-4-use-case-create-topic-alternate-flow-2}
 
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-create-topic-sequence-diagram}
+    \caption{Σενάριο χρήσης 2 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-create-topic-base-flow-sequence-diagram}
+\end{figure}
+
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-create-topic-alternate-flow-1-sequence-diagram}
+    \caption{Σενάριο χρήσης 2 - Διάγραμμα εναλλακτικής ροής 1}
+    \label{figure:3-4-use-case-create-topic-alternate-flow-1-sequence-diagram}
+\end{figure}
+
 % ===== =====
 % Use case 3
 % ===== =====
@@ -146,6 +167,34 @@
 {Σενάριο χρήσης 3 - Βασική ροή}
 {3-4-use-case-fetch-topic-base-flow}
 
+\useCaseAlternateFlowTable
+{1}
+{Το θέμα περιέχει ψηφοφορία.}
+{Εφόσον το θέμα που ανακτήθηκε στη γραμμή 1 της Βασικής Ροής περιέχει ψηφοφορία ανακτώνται οι πληροφορίες της.}
+{
+    1 & - & Το σύστημα ανακτά τα μηνύματα του θέματος αντιγράφοντας τις προσωπικές βάσεις OrbitDb των συγγραφέων. \\ [0.5ex]
+    2 & - & Το σύστημα ανακτά την ψηφοφορία από το blockchain. \\ [0.5ex]
+    3 & - & Το σύστημα ανακτά τις πληροφορίες της ψηφοφορίας αντιγράφοντας την προσωπική βάση OrbitDb του συγγραφέα. \\ [0.5ex]
+    4 & - & Το σύστημα επιβεβαιώνει τις πληροφορίες της ψηφοφορίας με βάση το hash που έχει ανακτηθεί από το blockchain. \\ [0.5ex]
+}
+{Το σενάριο χρήσης τερματίζεται.}
+{Σενάριο χρήσης 3 - Εναλλακτική ροή 1}
+{3-4-use-case-fetch-topic-alternate-flow-1}
+
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-fetch-topic-sequence-diagram}
+    \caption{Σενάριο χρήσης 3 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-fetch-topic-base-flow-sequence-diagram}
+\end{figure}
+
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-fetch-topic-alternate-flow-1-sequence-diagram}
+    \caption{Σενάριο χρήσης 3 - Διάγραμμα εναλλακτικής ροής 1}
+    \label{figure:3-4-use-case-fetch-topic-alternate-flow-1-sequence-diagram}
+\end{figure}
+
 % ===== =====
 % Use case 4
 % ===== =====
@@ -184,6 +233,13 @@
 {Σενάριο χρήσης 4 - Εναλλακτική ροή 1}
 {3-4-use-case-create-post-alternate-flow-1}
 
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-create-post-sequence-diagram}
+    \caption{Σενάριο χρήσης 4 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-create-post-base-flow-sequence-diagram}
+\end{figure}
+
 % ===== =====
 % Use case 5
 % ===== =====
@@ -207,6 +263,13 @@
 {Σενάριο χρήσης 5 - Βασική ροή}
 {3-4-use-case-vote-in-poll-base-flow}
 
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-vote-in-poll-sequence-diagram}
+    \caption{Σενάριο χρήσης 5 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-vote-in-poll-base-flow-sequence-diagram}
+\end{figure}
+
 % ===== =====
 % Use case 6
 % ===== =====
@@ -228,4 +291,11 @@
 }
 {Το σύστημα ανανεώνει τις ψήφους του μηνύματος.}
 {Σενάριο χρήσης 6 - Βασική ροή}
-{3-4-3-4-use-case-vote-post-base-flow}
+{3-4-use-case-vote-post-base-flow}
+
+\begin{figure}[H]
+    \centering
+    \input{tikz/chapter-3/3-4-use-case-vote-post-sequence-diagram}
+    \caption{Σενάριο χρήσης 6 - Διάγραμμα βασικής ροής}
+    \label{figure:3-4-use-case-vote-post-base-flow-sequence-diagram}
+\end{figure}
diff --git a/packages.tex b/packages.tex
index d7a03a5..ffa4d1e 100644
--- a/packages.tex
+++ b/packages.tex
@@ -25,6 +25,9 @@
 \usepackage{tabularx}							% Support for break lines inside table cells
 \usepackage{booktabs}							% Useful table styling commands
 
+% --- TikZ and UML diagrams
+\usepackage{pgf-umlsd}
+
 % --- Bibliography ---
 \usepackage[sorting=none]{biblatex}	% Make sure to have biber as default bibliography tool (e.g. in Texstudio's build configuration)
 
diff --git a/thesis.pdf b/thesis.pdf
index 25671b0..7113f15 100644
Binary files a/thesis.pdf and b/thesis.pdf differ
diff --git a/tikz/chapter-3/3-4-use-case-create-post-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-create-post-sequence-diagram.tex
new file mode 100644
index 0000000..8b21908
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-create-post-sequence-diagram.tex
@@ -0,0 +1,19 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[3]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Create post}{concordia}{Post creation form}
+    \end{call}
+
+    \begin{call}{actor}{Submit}{concordia}{Updated topic page}
+
+        \begin{call}{concordia}{Create post}{eth}{New post ID}
+        \end{call}
+
+        \begin{call}{concordia}{Save post information}{orbit}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-create-topic-alternate-flow-1-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-create-topic-alternate-flow-1-sequence-diagram.tex
new file mode 100644
index 0000000..11dfa45
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-create-topic-alternate-flow-1-sequence-diagram.tex
@@ -0,0 +1,28 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[4]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Create topic}{concordia}{Topic creation form}
+    \end{call}
+
+    \begin{call}{actor}{Add poll}{concordia}{Poll creation form}
+    \end{call}
+
+    \begin{call}{actor}{Submit}{concordia}{New topic-poll page}
+
+        \begin{call}{concordia}{Create topic}{eth}{New topic ID}
+        \end{call}
+
+        \begin{call}{concordia}{Add poll to topic}{eth}{}
+        \end{call}
+
+        \begin{call}{concordia}{Save topic information}{orbit}{}
+        \end{call}
+
+        \begin{call}{concordia}{Save poll information}{orbit}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-create-topic-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-create-topic-sequence-diagram.tex
new file mode 100644
index 0000000..9a4a269
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-create-topic-sequence-diagram.tex
@@ -0,0 +1,19 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[4]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Create topic}{concordia}{Topic creation form}
+    \end{call}
+
+    \begin{call}{actor}{Submit}{concordia}{New topic page}
+
+        \begin{call}{concordia}{Create topic}{eth}{New topic ID}
+        \end{call}
+
+        \begin{call}{concordia}{Save topic information}{orbit}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-fetch-topic-alternate-flow-1-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-fetch-topic-alternate-flow-1-sequence-diagram.tex
new file mode 100644
index 0000000..e7b0aef
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-fetch-topic-alternate-flow-1-sequence-diagram.tex
@@ -0,0 +1,41 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[2]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Fetch topic}{concordia}{Topic}
+
+        \begin{call}{concordia}{Get topic}{eth}{Topic}
+        \end{call}
+
+        \begin{call}{concordia}{Get poll}{eth}{Poll}
+        \end{call}
+
+        \begin{call}{concordia}{Get topic post IDs}{eth}{Post IDs}
+        \end{call}
+
+        \begin{call}{concordia}{Get posts loop}{concordia}{Posts}
+
+            \begin{call}{concordia}{Get post}{eth}{Post}
+            \end{call}
+
+        \end{call}
+
+        \begin{call}{concordia}{Retrieve topic information}{orbit}{}
+        \end{call}
+
+        \begin{call}{concordia}{Retrieve poll information}{orbit}{}
+        \end{call}
+
+        \begin{call}{concordia}{Validate poll information}{concordia}{}
+        \end{call}
+
+        \begin{call}{concordia}{Retrieve posts information loop}{concordia}{Posts information}
+
+            \begin{call}{concordia}{Retrieve post information}{orbit}{}
+            \end{call}
+
+        \end{call}
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-fetch-topic-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-fetch-topic-sequence-diagram.tex
new file mode 100644
index 0000000..15f0b95
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-fetch-topic-sequence-diagram.tex
@@ -0,0 +1,32 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[2]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Fetch topic}{concordia}{Topic}
+
+        \begin{call}{concordia}{Get topic}{eth}{Topic}
+        \end{call}
+
+        \begin{call}{concordia}{Get topic post IDs}{eth}{Post IDs}
+        \end{call}
+
+        \begin{call}{concordia}{Get posts loop}{concordia}{Posts}
+
+            \begin{call}{concordia}{Get post}{eth}{Post}
+            \end{call}
+
+        \end{call}
+
+        \begin{call}{concordia}{Retrieve topic information}{orbit}{}
+        \end{call}
+
+        \begin{call}{concordia}{Retrieve posts information loop}{concordia}{Posts information}
+
+            \begin{call}{concordia}{Retrieve post information}{orbit}{}
+            \end{call}
+
+        \end{call}
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-sign-up-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-sign-up-sequence-diagram.tex
new file mode 100644
index 0000000..0220bf1
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-sign-up-sequence-diagram.tex
@@ -0,0 +1,26 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[4]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+    \newinst[1]{orbit}{:OrbitDb}{}
+
+    \begin{call}{actor}{Sign up}{concordia}{Sign up form}
+    \end{call}
+
+    \begin{call}{actor}{Submit}{concordia}{Personal information form}
+
+        \begin{call}{concordia}{Create user}{eth}{New user ID}
+        \end{call}
+
+        \begin{call}{concordia}{Create databases}{orbit}{}
+        \end{call}
+
+    \end{call}
+
+    \begin{call}{actor}{Submit}{concordia}{Home page}
+
+        \begin{call}{concordia}{Save personal information}{orbit}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-vote-in-poll-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-vote-in-poll-sequence-diagram.tex
new file mode 100644
index 0000000..2fb9620
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-vote-in-poll-sequence-diagram.tex
@@ -0,0 +1,12 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[3]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+
+    \begin{call}{actor}{Submit poll vote}{concordia}{Updated topic page}
+
+        \begin{call}{concordia}{Add poll vote}{eth}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}
diff --git a/tikz/chapter-3/3-4-use-case-vote-post-sequence-diagram.tex b/tikz/chapter-3/3-4-use-case-vote-post-sequence-diagram.tex
new file mode 100644
index 0000000..404c21b
--- /dev/null
+++ b/tikz/chapter-3/3-4-use-case-vote-post-sequence-diagram.tex
@@ -0,0 +1,12 @@
+\begin{sequencediagram}
+    \newthread{actor}{Actor}{}
+    \newinst[3]{concordia}{:Concordia}{}
+    \newinst[2]{eth}{:Ethereum}{}
+
+    \begin{call}{actor}{Submit post vote}{concordia}{Updated topic page}
+
+        \begin{call}{concordia}{Add post vote}{eth}{}
+        \end{call}
+
+    \end{call}
+\end{sequencediagram}