The idiopathic inflammatory myopathies, or myositis syndromes, are heterogeneous autoimmune diseases defined by chronic muscle inflammation of unknown cause. They likely develop after the interaction of genetic and environmental risk factors in the absence of protective factors. The known genetic risk and protective factors are common alleles at polymorphic immune response loci and vary depending on phenotype. Furthermore, genetic associations are stronger with phenotypes defined by clinical features and autoantibodies than with myositis patients as a whole. Genetic factors for myositis also vary by age of onset, ethnicity, and environmental exposure group. Of interest, risk genes for one phenotype are often protective for another, possibly explaining the mutual exclusivity of many myositis subgroups. International collaborations using genome-wide association studies are needed to identify additional genes, gene-gene, and gene-environment interactions, all of which have pathogenic, therapeutic, and preventative implications for these increasingly recognized disorders.