Background:Collagen is the most abundant protein in the body and the main structural component of the skin. Objective:To provide a review of the histopathology of collagen alterations and to propose a classification with the most important types of collagen anomalies in dermatopathology. The authors describe some of the main morphological clues of collagen anomalies for specific diagnosis of some cutaneous inflammatory and neoplastic conditions. Methods:The authors review histopathologic collagen anomalies, concerning both morphology and disposition in some inflammatory and neoplastic cutaneous conditions, and they review previous terminology and proposed a classification of the most important types of collagen anomalies that can be seen in dermatopathological practice. Results:Collagen anomalies in skin can be classified into lamellar fibrosis, sclerosis, and "balls" and "rings" of collagen. Lamellar fibrosis presents as long and thin collagen bundles forming a delicate network, which can be disposed in a parallel pattern, onion-bulb-like pattern, and storiform pattern. Sclerosis is characterized by large, thick, and eosinophilic bundles of collagen, which may present as a homogenous-diffuse pattern or as individual thick bundles of collagen with few or abundant number of fibroblasts between them. Finally, the authors propose the terms "balls" and "rings" of collagen. The term "balls" of collagen stands for thick, homogenous, eosinophilic, globular collagen bundles, with no distinguishable individual composing fibers, which include the floating sign and the free-floating sign. The term "rings" of collagen is characterized by sclerotic collagen arranged in a homogenous rimming pattern around vessels without independent fibers in its composition. Conclusions:Collagen anomalies may be important clues to establish specific clues for specific diagnoses in dermatopathology.