This chapter reviews the family studies supporting the role of genetics and recent molecular genetic results. Mapping studies using linkage and association methods have had modest success to date despite difficulties in replication between studies. Linkage studies have shown the best support for chromosomal regions: 6q, 8q, 9p, 13q, 14q, and 22q. Several candidate genes first identified in studies of schizophrenia have shown reproducible association in bipolar disorder. Genome-wide association studies (GWAS) have been successful in identifying a few genes with small effects on risk. The data overall suggest a high level of both genic and allelic heterogeneity, as well as, a complex mode of inheritance. The coming availability of economical whole genome sequencing promises availability of complete genomic information. This, and large samples now being collected, may provide the datasets necessary to unravel the genetic complexities of this illness.