The factors determining the interaction with biological material of radiowaves and microwaves at the macroscopic, cellular and molecular levels are briefly discussed in terms of the relevant physical parameters for electromagnetic radiation in the frequency range 10 MHz–300GHz. This is followed by a summary of the results of the reported animal andin vitroexperiments, and it is concluded that, with the possible exception of a few cases awaiting further investigation, the biological effects of radiowaves and microwaves at these frequencies have a thermal basis for their explanation. The implication for human exposure of what is known in respect of fundamental mechanisms of interaction and the results of animal experiments are discussed, and it is explained why the present exposure limit of 10mW\cm2requires modification at the lower end of this frequency range.