In this chapter, we consider the issue of validation in the context of formal software development. Although validation is a standard practice in all industrial software development processes, this activity is somehow less well addressed within formal methods. As the needs for formal languages, tools and environments are increasing in producing real-life software, the validation issue must be addressed. In this chapter, we discuss what the place of validation within formal methods, what specific issues there are associated with formal methods as far as validation is concerned, and what tools can be used in this regard. We then present a few examples of the usefulness of validation from the case studies we have developed. The chapter is concluded with a few open research problems associated with validation and future work.