Atopic dermatitis (eczema) is a complicated skin condition. In fact, it represents many different underlying problems. These include abnormalities in the skin barrier, the immune system and the ability to handle different bacteria. Despite many recent gains in the understanding of eczema, a lot needs to be learned. There is little evidence to select between some of the older treatments that are available now. Even the newest targeted therapy does not clear most patients' skin. In order to improve the understanding of eczema, how to treat it now, and to enable discovery of new treatments, the investigators plan to enroll patients into a large study at three centres in Canada (University of Toronto, McGill University and University of British Columbia). The study is called the Canadian Atopic Dermatitis Cohort for Translational Immunology and Imaging (CACTI). In this study, the investigators will collect data on how severe participants' eczema is and what treatments they use. For participants who give permission, the investigators will collect blood and skin biopsy samples to study the mechanisms underlying their disease. The investigators will test to see which form of phototherapy (Broadband or Narrowband Ultraviolet B phototherapy) works best for eczema in a randomized clinical trial. The investigators will also use advanced imaging techniques to visualize the skin. This trial registry entry, including the description of the treatment arms and outcomes, refer to the nested BROadband vs Narrowband photoTherapy for Eczema clinical trial.