Psoriasis is a common inflammatory, hyperproliferative skin disorder that affects 1% to 2% of the population of Western Europe and the United States. Because the clinical presentation of psoriasis is varied, many times the definitive diagnosis depends on the histological examination. However, the histological changes of psoriasis are as varied as the clinical presentations. Therefore, a combination of histopathologic features must be present for the diagnosis of psoriasis to be made. In this article, we review the clues for the histopathologic diagnosis of each type and stage of psoriasis, such as psoriasis vulgaris, pustular psoriasis, and erythrodermic psoriasis, as well as the histopathologic differential diagnosis of these entities.