In the following we describe the building blocks of actin laments, the assembly and disassembly of laments, their kinetics, regulation, as well as lament bundles, network structures, and their mechanical properties.Actin itself is considered the most dynamic of the three cytoskeletal proteins capable of strong structural changes in the time scale of minutes, thus determining the shape of a cell.A single actin filament consists of actin monomers, called globular actin. Afterwards, it forms stable dior trimers and, nally, laments elongate by addition of monomers.Hydrolysis of ATP to ADP leads to a distinction between the fast growing end.One such control element is prolin.For the generation of a dense dendritic network not only nucleation factors, but also capping proteins are needed to restrict the elongation of the actin ends. The number of nodes is important for the mechanical properties of the generated network and consequently the Fenoprofen calcium hydrate elastic modulus scales with the mesh size M by M. Myosin is responsible for the contractility of antiparallel actin structures using ATP hydrolysis as the energy supply. These contractile structures are mainly responsible for the retraction of the cell rear for productive movement, but also for transmitting forces to the surrounding extracellular matrix.Interestingly, myosin II motor activity alone is insucient to produce contractility.Single myosin II hexamers are unipolar and thus ineective in generating contractile forces, but when assembled into bipolar minilaments they are highly processive and capable of generating forces by pulling on antiparallel actin laments. Myosin II can be activated via phosphorylation of the regulatory light chain. Another type of regulation works via the phosphorylation of the myosin heavy chain, utilizing myosin heavy chain kinases, inhibiting minilament assembly or dissociating existing minilaments. The switch between those two activation states inuences the contractility of the respective actomyosin network.Crosslinked actin bundles and networks largely control shape, mechanical integrity, and contractility of a cell. Generally, Chlorothiazide crosslinkers do not inuence actin assembly. Crosslinkers bind actin laments based on their own size and the position of their bindingsites in dierent distances, ranging from nm for mbrin to nm for lamin, and thus determine the density of the resulting actin structure. Additionally, the speed of actin polymerization inuences the presence of crosslinkers in the resulting network, supposedly by crowding eects, thus excluding larger crosslinkers like actinin in quickly polymerizing laments. If the formed actin structures are subject to a force that acts on a longer time scale than the binding time of the crosslinkers itself, a reorganization of crosslinkers and a subsequent shape change of the bundle occurs. This time scale depends on the type of crosslinker and its binding and unbinding time, which can be in the order of seconds and the presence of crosslinkers generally increases the elastic part of the viscoelastic answer to external stress.If actin laments are bundled by crosslinkers the laments inside the bundle can either be oriented in parallel or antiparallel, meaning that ends of neighboring laments are pointing in the same or the opposite direction.Parallel actin bundles are found amongst others in lopodia, while antiparallel bundles are mostly found in stress bers.