The reconstruction of the background spectrum is based on a decomposition of the initial spectrum into components which are due to the radiation from naturally occurring radionuclides on the Earth’s surface, cosmic rays, and radiation from the spectrometer itself. For the spectra of the naturally occurring radionuclides, the response functions of the detector are calculated for 238 U, 232 Th, and 40 K in the soil; the instrumental and cosmic-ray background is determined by means of aircraft flights at a large altitude. The method has been tested under real conditions and has given satisfactory results for the evaluation of the soil content of naturally occurring radionuclides and 137 Cs from global fallout. The method will increase considerably the sensitivity and accuracy of the aerial gamma-ray surveys for the detection of technogenic sources of radiation, whose activity is comparable to that of naturally occurring radionuclides and which have no distinct isolated photopeaks in the observed spectrum. One of the principal components determining the minimum detectable activity and the accuracy with which technogenic radioactive sources and contamination are detected during airborne surveying is the background radiation, including the radiation from naturally occurring radionuclides, the residual background of the spectrometer, and cosmic radiation. The contribution of radiation from naturally occurring radionuclides ‐ 40 K and the daughter products of 238 u and 232 Th decay ‐ predominates. The variation of the local relief, the content of naturally occurring radionuclides in soil and air, and the presence of a housing development lead to large variations of the background radiation spectrum; this makes it difficult to detect technogenic radiation, especially in the absence of total absorption peaks which stand out clearly in the measured spectrum. The method developed for reconstructing the background spectrum due to the radiation from naturally occurring radionuclides is based on calculation of the response functions of a detector for 40 K radiation and the daughter products of 238 U and 232 Th decay and optimizing their contribution to the total spectrum by the least-squares method. The contribution of the radiation from naturally occurring radionuclides is evaluated according to the count rate in several energy windows in the high-energy part of the spectrum, where the radiation from technogenic nuclides is, as a rule, absent. The contribution of cosmic radiation and the characteristic radiation of the detector to the measured spectrum is determined during flights at a high altitude where this contribution predominates. Least-Squares Reconstruction of a Spectrum. This method is based on representing the reconstructed spectrum as a superposition of components [1]. If the spectra of the individual components are known, the total spectrum can be represented with normalization coefficients, which, in turn, can be determined using the least-squares method by minimizing the expression