This chapter focuses on the perceptual mechanisms that enable and facilitate entrainment to the temporally regular sounds of music. It discusses possible unifying principles underlying functions of entrainment in the action range for human movement, where entrainment occurs most readily and with greatest precision. The chapter reviews timing constraints imposed by our biology and describes the metrical organization of time that is employed by music as well as our perceptual system. It argues that one main function of metrical structure is to circumvent temporal constraints inherent in the neural system. The chapter considers the function of metrical structure from the point of view of its efficacy at inducing the volitional underpinnings of entrainment, or groove. Although empirical studies do not support an association between groove and microtiming, the notion that temporal patterning might in fact facilitate entrainment is consistent with our functional perspective.