This book is an introduction to theoretical computer science, covering topics such as formal languages, automata theory, computability theory, and complexity theory. It provides a comprehensive overview of the foundational concepts, including regular languages and finite automata, context-free languages and pushdown automata, Turing machines and computability, and time and space complexity classes. The book also covers important theorems and results, such as the Pumping Lemma, the Church-Turing thesis, Godel's Incompleteness Theorem, and NP-completeness. It is written in a clear and concise manner, making it accessible to students and researchers with a basic understanding of discrete mathematics and programming. This book serves as an essential guide for anyone interested in the fundamental concepts of theoretical computer science.