Regression is an important histopathological parameter reported for the diagnosis of primary cutaneous melanoma. Histological regression is defined by The College of American Pathologists as the replacement of tumour cells by lymphocytic inflammation, with attenuation of the epidermis, and nonlaminated dermal fibrosis with inflammatory cells, melanophagocytosis and telangiectasia. Histological regression may be reported as absent versus present and, if present, as complete, partial or segmental. The stages of histological regression are early, intermediate and late, depending on the extent of histological inflammation and fibrosis. Regression occurs when the host's immune system attacks primary melanocytic tumour cells via tumour-infiltrating lymphocytes, resulting in fibrosis. The immunological mechanisms driving complete, partial and segmental regression may vary. In this first part of this two-part review, we review the history, histological criteria and pathogenesis of regression in primary cutaneous melanoma, while in Part 2 we will review the effect of histological regression on prognosis, evaluation and management.