Space travellers are irradiated with cosmic rays to a dose rate considerably higher than that received on earth. In order to make sensible judgements about space exploration, the risks to health of such radiation need to be assessed. Part of the assessment of risk is to allow for the enhanced biological effectiveness of high LET radiations with respect to others. In space the high LET radiations of concern are high energy neutrons and charged particles. At the doses and dose rates encountered in space, the important risk is the induction of cancer in the astronauts. For this biological end-point there is no direct human evidence for the relative effectiveness of these radiations. There are some data for neutrons for cancer and life-shortening in laboratory animals but these are for fission spectra neutrons, which are of lower energy than those encountered in space. There is a small amount of data for protons and high energy heavier charged particles. The remaining evidence comes from cellular experiments observing chromosome aberrations and gene mutations. From this sparse information, pragmatic choices need to be made for application to protection in space. The data are reviewed and the bases for the pragmatic choices discussed.