Computers have become an intricate part of lives now, but to become a fully qualified computer engineer, a deeper knowledge of the science and theory of computing is necessary. Before going into the practical applications of a subject, understanding of theory is essential. Theoretical computer science is a division that delves into the principles of computing. It is an abstraction of the science of computing, a mathematical analysis of theories and practices. Theoretical computer science is generally divided into three areas of study - automata, computability and computational complexity. The automata section is the study of mathematical models of computing machines called automata or abstract machines. The second section, computability, focuses on the sort of problems that can be solved by computers. It examines the limitations of computers, explores the boundaries of what can be achieved by computing machines. The third part, computational complexity theory is concerned with classifying computational problems according their levels of complexity. Theory Of Computer Science is the third edition of a popular textbook on theoretical computer science. There are many enhancements in this new edition. It covers the principle of induction and the pigeonhole principles in more depth, and also includes a rigorous proof of Kleene's theorem.  It has an expanded section on Turing Machines. This includes coverage of TM construction and multitape and non-deterministic Turing Machines. It includes new chapters on decidability, complexity theory, recursively enumerable languages, and NP- complete problems. There is an entire new section on quantum computing too. Theory Of Computer Science consists of many examples and exercises, and there are numerous objective questions at the end of each chapter. Solutions for the chapter-end exercises are provided at the end of the book. It is specifically aimed at engineering students who are pursuing their undergraduate or postgraduate courses in computer science and engineering.