The action of electromagnetic applicators in hyperthermic cancer therapy is investigated, with specific reference to invasive wire probes and noninvasive slots, along with their ability to heat biological tissue at depth without creating unwanted hot spots. An analysis based on spherical eigenfunctions enables useful probe configurations to be analysed and the field penetration depth calculated as a function of frequency and probe size. Alternative methods for assessing power flow in the tissue are considered. The use of a water bolus for impedance matching and isolating hot spots is examined, along with a comparison of the performance for two types of probe shape. Computed results show that the ability of a given probe to dispense heat at depth is, on the whole, insensitive to the applied frequency, although performance is worsened for very short probes. Approximate calculations for a noninvasive planar applicator have enabled heating patterns in the tissue to be examined and confirm that good beam characteristics are formed only a small distance into the tissue, but near-field hot spots are again a consideration. It is concluded for both of the applicators analysed here that lowering the frequency will not result in a significantly increased penetration depth due to near-field effects, and the clinical inference is noted.