Yes / Photobiomodulation is reported to positively influence hair regrowth, wound healing, skin rejuvenation, and psoriasis. Despite rapid translation of this science to commercial therapeutic solutions, significant gaps in our understanding of the underlying processes remain. The aim of this review was to seek greater clarity and rationality specifically for the selection of optical parameters for studies on hair regrowth and wound healing. Our investigation of 90 reports published between 1985-2015 revealed major inconsistencies in optical parameters selected for clinical applications. Moreover, poorly understood photoreceptors expressed in skin such as cytochrome c oxidase, cryptochromes, opsins, may trigger different molecular mechanisms. All this could explain the plethora of reported physiological effects of light. To derive parameters for optimal clinical efficacy of photobiomodulation, we recommend a more rational approach, underpinning clinical studies with research of molecular targets and pathways using well-defined biological model systems enabling easy translation of optical parameters from in vitro to in vivo. Furthermore, special attention needs to be paid when conducting studies for hair regrowth, aiming for double-blind, placebo-controlled randomized clinical trials as the gold standard for quantifying hair growth. / European Marie-Curie Actions Programme, Grant agreement no.: 607886