Our research focuses on the study of human T cell responses in skin and other peripheral tissues, including T cell function in healthy skin, impaired T cell migration and responses in skin cancers, and maladaptive T cell activation in inflammatory skin diseases and cutaneous lymphomas. We found that healthy adult human skin contains approximately 20 billion memory T cells, twice as many as are present in the circulation, leading to the discovery of resident memory T cells in skin. Nonrecirculating resident memory T cells play critical roles in defending skin against infection, but aberrant activation of these can lead to inflammatory diseases such as psoriasis and malignant transformation can give rise to cutaneous lymphomas . Skin cancers, including squamous cell carcinoma, melanoma and Merkel cell carcinoma, have evolved mechanisms for excluding or silencing resident memory T cells. Our studies, driven by observations in patients and carried out on human tissues, have the goals of developing novel therapies for skin disease and advancing our understanding of human immune responses.