Although circadian rhythms in mammalian physiology and behavior are dependent 
upon a biological clock in the suprachiasmatic nuclei (SCN) of the hypothalamus, 
the molecular mechanism of this clock is in fact cell autonomous and conserved 
in nearly all cells of the body. Thus, the SCN serves in part as a "master 
clock," synchronizing "slave" clocks in peripheral tissues, and in part directly 
orchestrates circadian physiology. In this chapter, we first consider the 
detailed mechanism of peripheral clocks as compared to clocks in the SCN and how 
mechanistic differences facilitate their functions. Next, we discuss the 
different mechanisms by which peripheral tissues can be entrained to the SCN and 
to the environment. Finally, we look directly at how peripheral oscillators 
control circadian physiology in cells and tissues.