The book focuses on three related areas in the theory of computation. The areas are modern cryptography, the study of probabilistic proof systems, and the theory of computational pseudorandomness. The common theme is the interplay between randomness and computation. The book offers an introduction and extensive survey to each of these areas, presenting both the basic notions and the most important (sometimes advanced) results. The presentation is focused on the essentials and does not elaborate on details. In some cases it offers a novel and illuminating perspective. The reader may obtain from the book 1. A clear view of what each of these areas is all above. 2. Knowledge of the basic important notions and results in each area. 3. New insights into each of these areas. It is believed that the book may thus be useful both to a beginner (who has only some background in the theory of computing), and an expert in any of these areas.