The term "development" is used in many different ways. This chapter begins, therefore, by briefly delineating the referential range which the term has throughout the book. We then propose some minimal criteria defining "development" (as opposed to, e.g., growth) and indicate its internal relation to the concept of developmental stages. Next we discuss the formal structure of development claims, taking as our prototype claims about specifically cognitive, or, as we prefer to say, conceptual development. The aim of this chapter is to explain the terminology used in this book and to introduce a conceptual apparatus through we can avoid certain unnecessary disputes in developmental theory.