Melasma is characterized by epidermal hyperpigmentation through increased melanogenesis in melanocytes. Some patients have dermal melanin but its amount is not significant and its distribution is very heterogeneous in the whole melasma lesional skin. It is likely that melasma is not a homogeneous disease and there are personal characteristics of patients with melasma. Melanocytes require the effective modulators of melanin synthesis produced by their neighboring keratinocytes or fibroblasts. The above discussion suggests that endogeneous or exogeneous stimuli may stimulate the microenvironment leading to release of various mediators that cause activation of melanocytes in the development of melasma. It is also possible that these stimuli directly stimulate the melanocytes. Melasma patients may have specialized melanocytes with intrinsic sensitivity to these stimuli.