One of the best known features of actin filaments is their helical structure. A number of essential properties emerge from this molecular arrangement of actin subunits. Here, we give an overview of the mechanical and biochemical implications of filament helicity, at different scales. In particular, a number of recent studies have highlighted the role of filament helicity in the adaptation to and the generation of mechanical torsion, and in the modulation of the filament's interaction with very different actin-binding proteins (such as myosins, cross-linkers, formins, and cofilin). Helicity can thus be seen as a key factor for the regulation of actin assembly, and as a link between biochemical regulators and their mechanical context. In addition, actin filament helicity appears to play an essential role in the establishment of chirality at larger scales, up to the organismal scale. Altogether, helicity appears to be an essential feature contributing to the regulation of actin assembly dynamics, and to actin's ability to organize cells at a larger scale.