Since its first description in 1930, the pathogenesis of pyoderma gangrenosum (PG) has remained obscure even as an ever-widening array of systemic diseases has been described in association with it. The histopathologic distinction of PG from other ulcerative processes with dermal neutrophilia is challenging and at times impossible. In consequence, when confronted with a biopsy from such a lesion, the pathologist has an obligation to obtain a full and detailed clinical history. In short, as a diagnosis of PG does not hinge exclusively upon the biopsy findings in isolation from other studies, a solid knowledge of the clinical features, the systemic disease associations and the differential diagnosis will help the pathologist to avoid diagnostic pitfalls or the generation of a report which is non-contributory to patient care. In this review, we describe in detail the different clinicopathologic forms of PG, summarize the diseases associated with this process in the literature and in our experience, and briefly review the treatment options.