We will present in this book the principles of securization of software-based equipment. At first, we will present the definition principles of security objectives. Secondly, we will show how it is possible to define a security architecture (including redundancy, diversification, error-detection techniques) on the basis of security objectives and how to identify objectives related to software programs. From software objectives, we will present the different securization techniques (fault detection, redundancy and quality control). The book will take the “certifiable system” aspects into account.