Psoriatic arthritis (PsA) occurs in approximately 30% of psoriasis patients. Understanding the similarities and differences in the etiology of these related diseases may highlight pathways for intervention and allow risk prediction in the future. Both are complex diseases in which environmental susceptibility factors trigger disease in genetically susceptible individuals. In recent years, genome-wide association studies have been highly successful in identifying genetic susceptibility factors for psoriasis. Most of the psoriasis loci tested so far are also associated with PsA. For example, associations of HLA-Cw*06 and the IL12B and IL23R genes are well-established with both conditions. More recently, analysis of psoriasis genome-wide association studies in a PsA subgroup has also implicated IL23A, TNFAIP3, and TNIP1 genetic variants as conferring risk to PsA. One study has suggested that late cornified envelope (LCE) gene polymorphisms are associated with psoriasis but not PsA. However, this finding was not confirmed by a second study. Similarly, association of the 5q31 gene region encompassing the IL13 gene has been reported with PsA but not psoriasis by one group, but this awaits confirmation in other series. Dedicated genome-wide association studies of PsA are underway and are likely to reveal further insights into why some patients with psoriasis develop arthritis whilst the majority do not.