This book introduces some of the most important results in theoretical computer science. The "gems" are central problems and their solutions from the areas of computability, logic, circuit theory, and complexity. The text presents complete proofs in understandable form, as well as previously open problems that have found a (perhaps unexpected) solution, complex proofs from bottom drawers, probabilistic constructions, and much, much more. With over 240 intriguing exercises (elegant solutions for which are supplied), the text also challenges the reader to do some active work.