This chapter is dedicated to a discussion about GO test problems and applications. Neither subject will be discussed in an exhaustive way. With respect to test problems, we have restricted our attention to a class of GO test problems, and discussed through it the issue of carefully selecting a subset of functions which is meaningful with respect to the kind of approach which has to be tested. Other GO test problems are not discussed in detail, but references are given to the existing literature and for some class of test problems the challenging aspects of the class are briefly discussed. With respect to the applications, listing all of them is quite hard due to the large number of publications in this field. Driven by our personal experience, we restrict our attention to four applications (namely molecular conformation, distance geometry, packing, and space trajectory planning problems) and give some pointers to a few others.