Actin is a highly conserved protein found in most eukaryotic cells, participating in numerous protein-protein interactions and regulating various cellular functions, including cell motility, shape maintenance, polarity, and transcription. The actin cytoskeleton is also targeted by pathogens for disruption or hijacking. This review discusses the structures of G- and F-actin and the interactions that control actin polymerization and disassembly. Actin monomers have a flat structure with two major domains (outer and inner) and two clefts: one for nucleotide binding and the other for actin-binding proteins (ABPs). The actin filament is a right-handed, two-chained helix with a twist of about 167° per residue. The G- to F-actin transition involves a propeller-like rotation of the outer domain, resulting in a flatter structure. The D-loop, which is involved in nucleotide sensing, plays a crucial role in this transition. Various ABPs, such as gelsolin, profilin, ADF/cofilin, and RPEL domains, regulate actin dynamics by binding to the target-binding cleft and influencing filament assembly and disassembly. Marine toxins like latrunculin A and B, and cytochalasin D, disrupt actin polymerization by binding to specific sites on the filament. The bacterial actin homologues MreB and ParM form straight, single-stranded filaments with left-handed helical symmetry. The G- to F-actin transition is influenced by the binding of proteins or small molecules, often through steric hindrance or competition for the D-loop binding site. Understanding these interactions is essential for elucidating the complex dynamics of the actin cytoskeleton and its role in cellular processes.Actin is a highly conserved protein found in most eukaryotic cells, participating in numerous protein-protein interactions and regulating various cellular functions, including cell motility, shape maintenance, polarity, and transcription. The actin cytoskeleton is also targeted by pathogens for disruption or hijacking. This review discusses the structures of G- and F-actin and the interactions that control actin polymerization and disassembly. Actin monomers have a flat structure with two major domains (outer and inner) and two clefts: one for nucleotide binding and the other for actin-binding proteins (ABPs). The actin filament is a right-handed, two-chained helix with a twist of about 167° per residue. The G- to F-actin transition involves a propeller-like rotation of the outer domain, resulting in a flatter structure. The D-loop, which is involved in nucleotide sensing, plays a crucial role in this transition. Various ABPs, such as gelsolin, profilin, ADF/cofilin, and RPEL domains, regulate actin dynamics by binding to the target-binding cleft and influencing filament assembly and disassembly. Marine toxins like latrunculin A and B, and cytochalasin D, disrupt actin polymerization by binding to specific sites on the filament. The bacterial actin homologues MreB and ParM form straight, single-stranded filaments with left-handed helical symmetry. The G- to F-actin transition is influenced by the binding of proteins or small molecules, often through steric hindrance or competition for the D-loop binding site. Understanding these interactions is essential for elucidating the complex dynamics of the actin cytoskeleton and its role in cellular processes.