Muscle energetics now emcompasses several related areas. Historically, the approach of energetics was largely phenomenological and primarily involved quantitation of muscle heat production and mechanical output in order to define performance characteristics. Based heavily on thermodynamics, it was thus independent of any particular model, and importantly set limits to which any theory of muscle contraction must conform. With the development of techniques for the direct assessment of A TP breakdown during contraction, such as rapid freezing and NADH-linked fluorescent techniques, the emphasis has shifted in recent years to what may be termed the kinetics of the actomy­ osin ATPase in situ. The focus is on how mechanical constraints affect this reaction, with the ultimate goal of understanding the mechanochemical transduction events at the crossbridge level. In parallel with these studies, muscle energetics now encompasses metabolism, particularly energy metabo­ lism, which focuses on the mechanisms underlying the coordination of metabolism and contractility. Excellent and comprehensive reviews of muscle energetics detailing these aspects for skeletal muscle are available (47, 97). For smooth muscle, the coupling of energy utilization and supply is of necessity (see below) very close; thus, these two areas are closely intertwined and are the focus for this chapter. The metabolism and energetics of smooth muscle have been comprehensively reviewed in the past several years (8, 68, 75). I will thus only briefly summarize what is widely accepted, emphasizing what is controversial.