The purpose of this work is to investigate experimentally and theoretically a range of problems encountered in calibration of -ray spectrometers (converting count rates to radioelement ground concentrations), for the natural and manmade radionuclides. For in-situ and aerial survey measurements, the form of radionuclide deposition with soil depth, aerial survey altitude, and detector spectral responses are important considerations when calibrating detector systems. A modification of spectral shape is apparent, owing to scattering and attenuation in the soil and air path between source and detector. A variety of depth profiles and detector configurations have therefore been considered, which are usually encountered in practice. It has been shown for the first time, that it is possible to reconstruct the full spectral response of a detector to calibrate a spectrometer from absolutely theoretical first principles. In doing so, one can avoid some of the problems inherent in experimental approaches. After overcoming technical and methodological problems, the work has been successful in all of its objectives. Experimental investigations of in-situ and aerial survey detectors serve as useful validation studies for theoretical models of the same detector types. The research therefore began with laboratory based measurements using point sources of radionuclides of interest. The acquisition of doped concrete calibration pads has enabled comparisons to be made with other facilities and spectrometers found world-wide. Small scale experimental simulations of detector responses at different altitudes have been made using the calibration pads and perspex absorbers. This extends and improves upon previous work done elsewhere, and uses more suitable absorber types. For the consideration of full energy responses only, analytical methods can be conveniently applied.