Mastocytosis is characterized by accumulation of pathologic mast cells in tissues. Most patients with mastocytosis experience mast cell activation symptoms in response to various triggers. The diagnosis of mastocytosis should be made from objective pathologic findings. Modern diagnostic criteria and classification of mastocytosis were proposed in 2000 by an international consensus group and formed the basis of the current World Health Organization (WHO) guidelines, which have been validated to correlate with prognosis and help selection of therapy. In this article, the WHO criteria for diagnosis and classification are summarized and practical aspects to avoid common pitfalls in diagnostic workup are discussed.