The protein actin forms filaments that provide mechanical support and driving forces for cell movement. Actin is essential for various biological processes, including sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing cells. These processes depend on interactions between actin monomers and filaments with numerous other proteins. The article discusses key questions in the field, such as identifying all the components of the actin system and understanding how they interact to function as a whole. It highlights the importance of quantitative measurements in live cells and computer simulations of mathematical models to advance our understanding of actin-based phenomena. The article also provides examples of biological processes that depend on actin, such as endocytosis, bacterial locomotion, cytokinesis, organelle transport, and cellular motility. Finally, it emphasizes the need for further research to understand the molecular mechanisms and regulatory pathways involved in actin-based functions.The protein actin forms filaments that provide mechanical support and driving forces for cell movement. Actin is essential for various biological processes, including sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing cells. These processes depend on interactions between actin monomers and filaments with numerous other proteins. The article discusses key questions in the field, such as identifying all the components of the actin system and understanding how they interact to function as a whole. It highlights the importance of quantitative measurements in live cells and computer simulations of mathematical models to advance our understanding of actin-based phenomena. The article also provides examples of biological processes that depend on actin, such as endocytosis, bacterial locomotion, cytokinesis, organelle transport, and cellular motility. Finally, it emphasizes the need for further research to understand the molecular mechanisms and regulatory pathways involved in actin-based functions.