Actin is a crucial protein involved in cell shape and movement. It forms filaments that provide mechanical support and drive cellular movement. Actin contributes to various biological processes, including sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing cells. These processes depend on interactions between actin and other proteins. Understanding actin-based biological phenomena requires identifying the molecules involved and defining their molecular mechanisms. Comparisons of live cell measurements with computer simulations can generate insights. Actin evolved from a common ancestor about 3 billion years ago and is essential for most cells. It provides mechanical support, tracks for intracellular movement, and force for cell movement. Many prokaryotes use actin relatives for shape and DNA movement. Eukaryotes have actin genes and myosin motor proteins. Actin and myosin were discovered in muscle cells in the 1940s and later found in other cells, revealing muscle filaments as a specialized example of a common system. Actin filaments are polar, with one end growing faster than the other. They bind nucleotides and undergo structural changes that prepare them for disassembly. Eukaryotic cells use over 100 accessory proteins to regulate actin. These proteins help maintain actin monomers, initiate polymerization, and regulate filament assembly and turnover. Actin filaments drive cell crawling, a feature of amoebae and animal cells. They also contribute to membrane vesicle internalization. Myosin motor proteins interact with actin filaments to produce two types of movements: force generation for cell contraction and cargo transport along filaments. Actin filaments are part of the cytoskeleton, providing mechanical properties and shapes to cells. They are involved in cell division, endocytosis, and sensing external forces. Actin patches are sites of plasma membrane internalization and involve complex molecular interactions. Bacteria can use actin filaments to form comet tails for propulsion. Cytokinesis involves actin filaments and myosin-II to pinch cells in two. Some organisms use different mechanisms for cytokinesis, such as membrane fusion. Actin cables and organelle transport involve myosin motors moving organelles along actin filaments. Cellular motility relies on actin filaments for movement, with Arp2/3 complex assembling branched filaments to generate force. The field of actin research focuses on understanding molecular mechanisms and interactions. Mathematical models and simulations are essential for interpreting complex biological processes. Continued research with model systems and systems-level genomics approaches will help define the molecular basis of actin-based functions. Technical advances in imaging and microscopy will aid in understanding actin systems, especially filament assembly and network formation.Actin is a crucial protein involved in cell shape and movement. It forms filaments that provide mechanical support and drive cellular movement. Actin contributes to various biological processes, including sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing cells. These processes depend on interactions between actin and other proteins. Understanding actin-based biological phenomena requires identifying the molecules involved and defining their molecular mechanisms. Comparisons of live cell measurements with computer simulations can generate insights. Actin evolved from a common ancestor about 3 billion years ago and is essential for most cells. It provides mechanical support, tracks for intracellular movement, and force for cell movement. Many prokaryotes use actin relatives for shape and DNA movement. Eukaryotes have actin genes and myosin motor proteins. Actin and myosin were discovered in muscle cells in the 1940s and later found in other cells, revealing muscle filaments as a specialized example of a common system. Actin filaments are polar, with one end growing faster than the other. They bind nucleotides and undergo structural changes that prepare them for disassembly. Eukaryotic cells use over 100 accessory proteins to regulate actin. These proteins help maintain actin monomers, initiate polymerization, and regulate filament assembly and turnover. Actin filaments drive cell crawling, a feature of amoebae and animal cells. They also contribute to membrane vesicle internalization. Myosin motor proteins interact with actin filaments to produce two types of movements: force generation for cell contraction and cargo transport along filaments. Actin filaments are part of the cytoskeleton, providing mechanical properties and shapes to cells. They are involved in cell division, endocytosis, and sensing external forces. Actin patches are sites of plasma membrane internalization and involve complex molecular interactions. Bacteria can use actin filaments to form comet tails for propulsion. Cytokinesis involves actin filaments and myosin-II to pinch cells in two. Some organisms use different mechanisms for cytokinesis, such as membrane fusion. Actin cables and organelle transport involve myosin motors moving organelles along actin filaments. Cellular motility relies on actin filaments for movement, with Arp2/3 complex assembling branched filaments to generate force. The field of actin research focuses on understanding molecular mechanisms and interactions. Mathematical models and simulations are essential for interpreting complex biological processes. Continued research with model systems and systems-level genomics approaches will help define the molecular basis of actin-based functions. Technical advances in imaging and microscopy will aid in understanding actin systems, especially filament assembly and network formation.