[en] Environmental radiation measurements are performed in the field to determine radionuclide composition, supplement calculated values of dose, and detect any changes from some normal value that can be attributed to a particular source. Instrument calibrations with laboratory sources, while necessary, are generally insufficient for these environmental applications. Account must be taken of the complex radiation field composition, low intensity and source-detector geometry, as well as of the instrument energy and angle responses. Transport calculations of the radiation field aid in the development of meaningful calibrations. Energy and angle spectra over the ground, the principal environmental source, have been calculated for different source distributions and gamma-ray energies, and verified experimentally. By determining detector responses to background components in this way, one can estimate any excess response to man-made sources. Operationally, consistency of field measurements with different instruments tends to assure good quality results. This paper describes measurement methods developed for studies of natural, nuclear-facility and nuclear-weapon radioactivity. Some of the methods were incorporated into a recent (US) NCRP report, Environmental Radiation Measurements. Possible new monitoring problems are identified. (author)