An appreciation of the salient physical properties of phototherapy and photochemotherapy treatment equipment should benefit any center constructing its own unit, or may influence the choice of a commercial system. These properties are discussed and different commercial units are listed. Fluorescent lamp arrays are the most commonly used source of ultraviolet radiation. The spectra of fluorescent lamps for PUVA are very similar, but there are clinically important variations between the emission spectra of different types of UVB lamp. The spatial uniformity and magnitude of irradiance will be affected by the number and distribution of tubes in a unit, and by the use of aluminium reflectors. It is usual to find such reflectors located behind an array of lamps, but horizontal reflectors positioned above and below enclosed vertical cabinets are often omitted. The measured irradiance in a unit depends on many factors, and specified irradiances must be compared with caution. Treatment should be prescribed by dose rather than time, and on-line integration of the irradiance to control the exposure should lead to more reproducible treatment. The ability of a unit to be fitted with both UVA and UVB lamps is a factor to be considered by clinical centers offering both PUVA and phototherapy treatment.