There have been very significant advances made over the past 20 years in our understanding of the pathogenesis of psoriasis underpinned by a combination of genetic discovery and immunological functional studies. Key to pathogenesis is the interplay between the epidermal barrier and innate immune mechanisms leading to activation of adaptive T-cell immune pathways which in turn lead to changes in skin homeostasis and inflammation. Specific signalling (e.g. NF-κB) and T-cell differentiation pathways (Th IL-17) have been identified which in turn have led to therapeutic advances. Pustular forms of the disease have a distinct genetic background.