**1. Introduction**

Psoriasis is not a merely skin disease but rather a multisystemic inflammatory disorder. As such, comorbidities, including but not limited to cardiovascular diseases and metabolic syndrome, play a significant role in therapeutic decision-making. Even though skin comorbidities in patients with psoriasis are common, they are usually neglected. The epidemiology studies and therapy-related literature dominate the recently published relevant data. The pathogenesis of comorbid diseases, either systemic or dermatologic, in patients with psoriasis remains unknown; however, shared inflammatory pathways, cellular mediators, genetic susceptibility, and common risk factors are hypothesized to be contributing elements.

Patients with psoriasis are more likely to have at least one other autoimmune disease [1], including vitiligo [2], alopecia areata [1], and autoimmune bullous diseases [3].

The clinical implications of the most frequent comorbid skin diseases associated with psoriasis are discussed in this chapter.
