In the first decade of the 21st Century a significant milestone was reached when the urban proportion of the world’s population of 6.6 billion passed 50%. This proportion will increase rapidly in the decades to come as parts of Asia and Africa become progressively less rural and more urban. Although urban areas occupy less than 3% of the planetary landmass, they are the foci of humans and economic activity. The climates that they generate are distinctive and represent unambiguous evidence of the anthropogenic climatic effect. This urban climate effect is a consequence of two related properties, land cover (form) and land use (function). Urbanisation replaces ‘natural’ surfaces with manufactured materials that are usually impervious and have distinctive thermal and radiative properties. In addition, the urban surface is both geometrically complex and highly diverse. These properties of form result in the formation of myriad microclimates caused by a number of climate drivers, including access to the sun and shelter from wind. Urban function refers to the human activities that generate waste heat, moisture and materials as a result of transportation, industrial production, energy consumption, and so on. These functions tend to have distinctive spatial and temporal emission patterns. Together, urban form and urban function generate urban climates at a hierarchy of scales.