The article reviews the recent advancements in understanding the cellular regulation and function of the Arp2/3 complex and formins, as well as the discovery of new nucleation factors in mammalian cells. Actin, a highly conserved protein in eukaryotic cells, plays a crucial role in various cellular functions such as morphogenesis, migration, cytokinesis, and membrane transport. The Arp2/3 complex is a key nucleator that can both nucleate and organize actin filaments into branched networks. Recent studies have identified several new nucleation-promoting factors (NPFs) such as WASH, WHAMM, and JMY, which stimulate Arp2/3 complex activity at distinct cellular locations. Formin proteins, on the other hand, are known for their ability to nucleate unbranched actin filaments and have diverse biochemical and cellular activities. The Spire, Cordon-bleu, and Leiromodin nucleators have also been discovered, revealing new ways to overcome the kinetic barriers to actin polymerization. The article discusses the structural and biochemical properties of these nucleators, their interactions with other proteins, and their roles in various cellular processes. It highlights the importance of these factors in understanding the dynamics of the actin cytoskeleton and their potential roles in disease.The article reviews the recent advancements in understanding the cellular regulation and function of the Arp2/3 complex and formins, as well as the discovery of new nucleation factors in mammalian cells. Actin, a highly conserved protein in eukaryotic cells, plays a crucial role in various cellular functions such as morphogenesis, migration, cytokinesis, and membrane transport. The Arp2/3 complex is a key nucleator that can both nucleate and organize actin filaments into branched networks. Recent studies have identified several new nucleation-promoting factors (NPFs) such as WASH, WHAMM, and JMY, which stimulate Arp2/3 complex activity at distinct cellular locations. Formin proteins, on the other hand, are known for their ability to nucleate unbranched actin filaments and have diverse biochemical and cellular activities. The Spire, Cordon-bleu, and Leiromodin nucleators have also been discovered, revealing new ways to overcome the kinetic barriers to actin polymerization. The article discusses the structural and biochemical properties of these nucleators, their interactions with other proteins, and their roles in various cellular processes. It highlights the importance of these factors in understanding the dynamics of the actin cytoskeleton and their potential roles in disease.