As in the LISP tradition, SMALLTALK is almost entirely written in itself. It offers important advantages such as large portability, dynamicity, a fully unified world, graphical user interface builders, connection to databases, powerful development tools, etc. In this paper we discuss the trait that underlies all these features: REFLECTION. We quote one of its definitions and in the first part of this paper go through the different reflective aspects of SMALLTALK. We expand five major aspects in detail: meta-operations, the classes/metaclasses model, the reified compiler, message sending and the behavioral representation through the reification of the executive stack frame of each process. We illustrate their use with significant applications, based both on our industrial and research experiences. In the second part of the paper, we introduce and fully developpre/post conditions in SMALLTALK, dealing with extensions of the model, the compiler, and the development environment.