In a lithographic projection apparatus a liquid supply system maintains liquid in a space between a final element of the projection system and the substrate with a liquid confinement system. The liquid supply system further comprises a de-mineralizing unit, a distillation unit and a UV radiating source for the purification of immersion liquid. Chemicals may be added to the immersion liquid for the inhibition of lifeform growth and components of the liquid supply system may be made of a material which is non-transparent to visible light such that growth of lifeforms is reduced.