Over the past 2 decades our knowledge about actin filaments has evolved from a rigid ''pearls on a string'' model to that of a complex, highly dynamic protein polymer which can now be analysed at atomic detail, To achieve this, exploring actin's oligomerization, polymerization, polymorphism, and dynamic behavior has been crucial to understanding in detail how this abundant and ubiquitous protein can fulfill its various functions within living cells. In this review, a correlative view of a number of distinct aspects of actin is presented, and the functional implications of recent structural, biochemical, and mechanical data are critically evaluated, Rational analysis of these various experimental data is achieved using an integrated structural approach which combines intermediate-resolution electron microscopy-based 3-D reconstructions of entire actin filaments with atomic resolution X-ray data of monomeric and polymeric actin. (C) 1997 Academic Press.