This thesis focuses on the coordination of proliferation and differentiation in embryonic and adult myogenesis. During development, we demonstrated that skeletal muscle progenitors interact with the differentiating myoblasts via the Notch pathway to maintain their pool. It has previously been established that p57 and p21 redundantly promote cell cycle exit in developing muscle and we showed that Notch and Myogenic Regulatory Factors act through muscle-specific regulation of p57. We then examined p21 and p57 in adult skeletal muscle stem cells, called satellite cells (SCs). Although absent from quiescent SCs, p21 and p57 are expressed upon activation (including proliferating myoblasts) and differentiation. p21-null myoblasts exhibited proliferation and differentiation defects in myofiber cultures, implicating p21 at the early activation phase. In vivo muscle regeneration studies with p21 mutants showed an early impact on the SC pool, while SCs and muscle structure were re-established by the end of regeneration. Since p57-deficient mice die at birth, we generated a conditional knock-out (KO) allele for postnatal studies using the loxP/Cre recombination system. With a ubiquitous Cre we observed developmental and perinatal phenotypes similar to previously described KO embryos. The new p57 allele also includes a β-galactosidase reporter and we showed that it recapitulates p57 expression profile in embryonic and adult tissues. Conditional ablation of p57 in adult SCs reduced myogenic differentiation in primary myoblast culture. Our work suggests that p21 and p57 are involved in adult myogenesis and cell cycle exit, working at the early steps of satellite cell activation.