Myositis, or idiopathic inflammatory myopathies (IIM), are heterogeneous diseases. The pathomechanisms of IIM can be caused by specific interactions between genetic and environmental risk factors, however, the details are not clear. This article reviews that have been made in our understanding of the studies of genetics in IIM, with particular focus on inclusion body myositis (IBM), immune–mediated necrotizing myopathies (IMNM), myositis specific autoantibodies (MSA) and PD–1 inhibitor–associated myopathies. The human leukocyte antigen (HLA) 8.1 ancestral haplotype (8.1 AH) is a key risk factor for major IIM phenotypes in Caucasian, and other genetic variants have been identified as risk factors in several IIM phenotypes. Owing to the rarity of IIM in Japan, there are few studies of genetic analysis. Since the HLA allele frequency varies greatly among ethnicity, it is necessary to proceed with research unique to Japanese.