Evaluation of dermatitis associated with propylene glycol application or ingestion remains a challenge. The research dealing with skin reactions to propylene glycol is revisited and new aspects for future research are outlined. Based on literature review and our own observations, we propose classifying skin reactions to propylene glycol into 4 mechanisms: (a) irritant contact dermatitis, (b) allergic contact dermatitis, (c) non-immunologic contact urticaria, and (d) subjective or sensory irritation. This concept allows a partial explanation of effects observed by different authors. Despite attempts to define objective criteria, biologically, histopathologically, or clinically, the distinction between irritant and allergic reactions remains unclear. Furthermore, the irritation threshold of propylene glycol, and likewise the optimal standard concentration in patch tests, is sub judice. Future studies on propylene glycol dermatitis should include repeated patch tests with serial dose dilutions, repeated open application tests/provocative use tests, oral challenge tests, and biopsies for a more complete evaluation of mechanisms and clinical significance.