An introduction to the use of microwaves for medical applications is given, with a comparison between different modalities for heating tissue, such as radio-frequency and laser based techniques. The background to the design and construction of applicators using finite element packages is also explained and this knowledge is demonstrated in the design, construction and testing of novel systems to treat psoriasis and varicose veins. The psoriasis treatment system is hyperthermia based, whilst the varicose veins treatment employs ablation, thus giving an insight into two different means of inducing a therapeutic effect using heat. Thermal analysis of the heat induced by the applicators is demonstrated by numerical solution of the diffusion equation by finite difference approximations and Green's function analysis, with the results being validated against bench models. The results of testing of the psoriasis system on three patients are given, including the first quantitative assessment of the relationship between skin temperature and blood perfusion levels for both psoriatic and healthy skin at hyperthermic temperatures. The varicose veins treatment is minimally invasive, and is the first microwave based treatment for varicose veins.