Tight coupling between biochemical and mechanical properties of the actin cytoskeleton drives a large range of cellular processes including polarity establishment, morphogenesis, and motility. This is possible because actin filaments are semi-flexible polymers that, in conjunction with the molecular motor myosin, can act as biological active springs or "dashpots" (in laymen's terms, shock absorbers or fluidizers) able to exert or resist against force in a cellular environment. To modulate their mechanical properties, actin filaments can organize into a variety of architectures generating a diversity of cellular organizations including branched or crosslinked networks in the lamellipodium, parallel bundles in filopodia, and antiparallel structures in contractile fibers. In this review we describe the feedback loop between biochemical and mechanical properties of actin organization at the molecular level in vitro, then we integrate this knowledge into our current understanding of cellular actin organization and its physiological roles. I. PREFACE ON ACTIN ORGANIZATION AND CELL MECHANICSAnimal cells (i.e., those without a cell wall) have the ability to change their shape to adapt to their environment, move through narrow spaces, divide, or allow exo-and endocytosis. The machinery of these shape changes relies on the assembly of proteins, in particular actin, a globular protein that polymerizes into filaments of different types of organization: branched and crosslinked networks, parallel bundles, and anti-parallel contractile structures (FIGURE 1A). These different architectures can be envisioned as a series of interconnected active springs and dashpots (green and red symbols in FIGURE 1B, respectively) that act as mechanical elements to drive cell shape changes and motility. The purpose of this review is to correlate recent progress in our understanding of the interplay between biochemical elements and mechanical properties. Instead of describing biochemical and mechanical properties separately, our main goal here is to address piece by piece the integrated feedback loop between biochemistry and mechanics.At the front of the cell, branched and crosslinked networks in a quasi two-dimensional sheet make up the lamellipodium, and are the major engine of cell movement since they push the cell membrane by polymerizing against it (FIGURE 1A, iii). Aligned bundles underlie filopodia that are the fingerlike structures at the front of the cell, important for directional response of the cell (FIGURE 1A, iv). A thin layer of actin, called the cell cortex, coats the plasma membrane at the back and sides of the cell, important for cell shape maintenance and changes (FIGURE 1A, i). The rest of the cell contains a three-dimensional network of crosslinked filaments interspersed with contractile bundles, including stress fibers that connect the cell cytoskeleton to the extracellular matrix via focal adhesion sites (FIGURE 1A, ii). Contraction in the cell is produced by the molecular motor protein myosin. Myosin assembles into anti...