Underlying our understanding of urban climates is an understanding of the urban surface and how it impacts the atmosphere above. Throughout his career, T.R. Oke has emphasized conceptual definitions of the urban surface and how they impact processes important to urban climate. In this presentation we review the conceptual definitions of the urban surface and show how our understanding and use of these definitions have led to important advances in urban climate in areas related to urban turbulence, source areas, heat storage, heat island interpretation and thermal anisotropy.