The established observations and unresolved questions in the assembly of myosin are outlined in this article. Much of the background information has been obtained in classical experiments using the myosin and thick filaments from vertebrate skeletal muscle. Current research is concerned with problems of myosin assembly and structure in smooth muscle, a broad spectrum of invertebrate muscles, and eukaryotic cells in general. Many of the general questions concerning myosin assembly have been addressed by a combination of genetic, molecular, and structural approaches in the nematode Caenorhabditis elegans. Detailed analysis of multiple myosin isoforms has been a prominent aspect of the nematode work. The molecular cloning and determination of the complete sequences of the genes encoding the four isoforms of myosin heavy chain and of the myosin-associated protein paramyosin have been a major landmark. The sequences have permitted a theoretical analysis of myosin rod structure and the interactions of myosin in thick filaments. The development of specific monoclonal antibodies to the individual myosins has led to the delineation of the different locations of the myosins and to their special roles in thick filament structure and assembly. In nematode body-wall muscles, two isoforms, myosins A and B, are located in different regions of each thick filament. Myosin A is located in the central biopolar zones, whereas myosin B is restricted to the flanking polar regions. This specific localization directly implies differential behavior of the two myosins during assembly. Genetic and structural experiments demonstrate that paramyosin and the levels of expression of the two forms are required for the differential assembly. Additional genetic experiments indicate that several other gene products are involved in the assembly of myosin. Structural studies of mutants have uncovered two new structures. A core structure separate from myosin and paramyosin appears to be an integral part of thick filaments. Multifilament assemblages exhibit multiple nascent thick filament-like structures extending from central paramyosin regions. Dominant mutants of myosin that disrupt thick filament assembly are located in the ATP and actin binding sites of the heavy chain. A model for a cycle of reactions in the assembly of myosin into thick filaments is presented. Specific reactions of the two myosin isoforms, paramyosin, and core proteins with multifilament assemblages as possible intermediates in assembly are proposed.