The cutaneous deposition disorders are a group of unrelated conditions characterized by the presence of either endogenous or exogenous substances within the dermis or the subcutis. Part I of this two-part series will focus on metabolic processes involved in the endogenous deposition in the various forms of amyloidosis, porphyria, colloid milium, and lipoid proteinosis. We will also review the clinical, histologic, biochemical, and ultrastructural findings relevant to each disorder. Basic mechanisms of pathogenesis, diagnostic modalities, and treatment options are also discussed. (J Am Acad Dermatol 1998;39:149-71.) Learning Objective: After reading this article, participants should be familiar with the deposition disorders. On the basis of the clinical and histologic findings discussed for each disorder, clinicians should be able to distinguish between these often-confusing entities.