Three of the four articles in this section deal with contractility and its relation to structure in a variety of cells. They are complemented by one reviewing a novel way of studying cell metabolism-a prerequisite of mo­ tion-by a nondestructive process. Goldman et al deal with the wide variety of filaments found in mam­ malian cells. In addition to filaments made up of proteins analogous to those found in the muscle cell, other filaments or fibers exist that differ in mor­ phology from actinand myosin-containing filaments. A number of proteins have been recognized as their constituents. The functional role of these filaments in maintaining cell structure and in cellular contractility-a fast growing field-is the main theme of the review. Tregear & Marston cover the more conventional contractile apparatus, i.e. that found in skeletal muscle. They review current views on the mecha­ nism by which the portion of the myosin molecule that carries the active site for the ATP hydrolysis interacts with actin. In the view of the large majority of workers in this field, myosin molecules interacting with actin form the so-called crossbridges, whose cyclic formation and breaking, cou­ pled to ATP hydrolysis, is the basis of force generation. Tregear & Marston present an up-to-date review on the topic, together with some suggestions of their own. While knowledge of the striated muscle system has reached a high level of development, information concerning smooth muscle has been lagging in many areas. Recent work has cleared up several uncertainties (particularly in regard to structure), has led to progress in energetics, and has brought to light some novel aspects of control. The chapter by Murphy is, therefore, a timely review of a rapidly developing field. The relation between cell metabolism and cell function, whether that function be muscle contraction, secretion, or the generation of an electrical signal, has been recognized as a fundamental problem of biology. Classical