The so called information society is being shaped by products and services that either contain software as the core element, are themselves software, or have been designed and manufactured with the help of sophisticated software applications. The new global environment in which these systems are being developed and deployed, in addition to other pressing issues, such as time-to-market and dependability, are pushing the skills of software developers and engineers to the limit. With few exceptions, those products and services will influence the social environment where they will be deployed, because people will operate them or be affected by them, or both. So another challenging issue is the ability of these systems to integrate with their social environment in a way that is perceived as beneficial, trustworthy and acceptable. Given these challenges, defining the requirements that establish what a future system should do, and validating that the system has achieved its goals, is an immense task. In this book the reader will find a detailed account of the challenges posed by the elicitation, specification, validation and management of requirements for the definition of this kind of application: from embedded software in cars to internet-based applications, COTS packages, health-care, and others.