Radioactivity in the environment includes naturally occurring as well as man-made radionuclides. Meaningful estimates of their hazard to health require much care in obtaining representative environmental samples, appropriate sample preparation as well as proper calibration of the measuring system. Any harm from the low absorbed doses of ionizing radiation typically received from environmental radioactivity would only occur after a latent period of several years. Of these, cancer induction is considered to be the most important. Risk factors have been derived by a number of internationally recognized organizations, but there are difficulties in extrapolating from observations at a high dose and dose rate and for the lifetime of exposed populations. Examples of measurement of222Ra,90Sr,125I,226Ra,129I and137Cs in the environment are given. Harmful effects from environmental radioactivity have mostly been indistinguishable above natural incidence. For health purposes there may be little need for further increases in sensitivity of detection but there is a continuing need to ensure the greatest accuracy in measurements to avoid magnifying the inevitable uncertainties in estimating radiation hazards.