Sensing Hotterdam recorded the temperature in 1,000 Rotterdam homes and at 300 public spaces in the summer of 2014 in order to outline the links between the urban heat island, the built environment and public health in the city of Rotterdam. The measured outdoor temperature readings point to a clear heat island effect in Rotterdam. Temperatures in homes are generally higher than those in the surrounding area, and also show a large degree of variation. Indoor temperatures are less affected by local outdoor temperatures than we had expected.