Although in the past, keratinocytes were considered simply as passive targets of immunological attack from infiltrating T lymphocytes, a number of studies have definitively demonstrated that keratinocytes actively participate in the cutaneous immune responses. Upon activation, keratinocytes express a plethora of cytokines, chemokines and accessory molecules, which can transmit both positive and negative signals to cells of innate and adaptive immunity. Dysregulation and abnormal expression of inflammatory mediators or their receptors in keratinocytes are relevant to the pathogenesis of chronic inflammatory skin diseases such as psoriasis, atopic dermatitis and allergic contact dermatitis.