A study of smooth classes whose generic structures have simple theory is carried out in a spirit similar to Hrushovski (Ann. Pure Appl. Logic 62 (1993) 147; Simplicity and the Lascar group, preprint, 1997) and Baldwin-Shi (Ann. Pure Appl. Logic 79 (1) (1996) 1). We attach to a smooth class K0; βΊ of ΓΏnite L-structures a canonical inductive theory TNat, in an extension-by-deΓΏnition of the language L. Here TNat and the class of existentially closed models of (TNat) β =T+; EX (T+), play an important role in description of the theory of the K0; βΊ -generic. We show that if M is the K0; βΊ -generic then M β EX (T+). Furthermore, if this class is an elementary class then Th(M ) = Th(EX (T+)). The investigations by Hrushovski (preprint, 1997) and Pillay (Forking in the category of existentially closed structures, preprint, 1999), provide a general theory for forking and simplicity for the nonelementary classes, and using these ideas, we show that if K0; βΊ , where βΊ β {6 ; 6 * }, has the joint embedding property and is closed under the Independence Theorem Diagram then EX (T+) is simple. Moreover, we study cases where EX (T+) is an elementary class. We introduce the notion of semigenericity and show that if a K0; βΊ -semigeneric structure exists then EX (T+) is an elementary class and therefore the L-theory of K0; βΊ -generic is near model complete. By this result we are able to give a new proof for a theorem of Baldwin and Shelah (Trans. AMS 349 (4) (1997) 1359). We conclude this paper by giving an example of a generic structure whose (full) ΓΏrst-order theory is simple.