In this thesis, a research on the radiological properties of soil in the Republic of Croatia is presented. This research is focused mostly on radionuclides that commonly appear in soil and which are the sources of ionizing radiation (α, β, and γ radiation). Of all the constituents of the environment, soil is generally the medium with the highest radionuclide concentrations. Radionuclides from the other media, air and water, may also end up in soil. Because of that, the presented research can be considered to be an indicator of the general state of the environmental radioactivity in Croatia, representing a starting point for an assessment of exposure of the population to ionizing radiation from the environment. Radionuclides that can be found in the environment, and which are mainly considered in this work, can be separated into two groups: naturally occurring radionuclides, and anthropogenic radionuclides. Primordial radionuclides, which form a majority of naturally occurring radionuclides, originate from the time before the formation of Earth, as they were produced in massive star explosions. These radionuclides have a half-life of a few billions years, hence they are present in the environment even today. The main representatives of the primordial radionuclides are 40K, 238U, 235U, and 232Th. The latter three of them are the first ones in their respective decay chains which undergo a series of around fifteen decays before ending up with a stable nucleus. Radionuclides which appear in the decay chains, for instance 226Ra, 210Pb, and 208Tl, are called secondary radionuclides, and they are also considered in this work. The remaining naturally occurring radionuclides, which are not so abundant, are produced by the interactions of cosmic radiation with Earth's atmosphere, and they are therefore termed cosmogenic radionuclides. These radionuclides primarily appear in air, and they have not been considered in this work. Anthropogenic radionuclides are produced by human activity. The main source of anthropogenic radionuclides are nuclear fission reactions that occur in nuclear reactors or in nuclear weapon explosions. In nuclear power plants, anthropogenic radionuclides are produced in large amounts. In nuclear accidents such as those which occurred in Chernobyl or Fukushima, considerable quantities of anthropogenic radionuclides may end up in the environment, where they pose a serious threat to people and other living organisms. Anthropogenic radionuclides which are found in soil at the territory of Croatia are 137Cs and 90Sr. The presence of 137Cs is mainly a consequence of the Chernobyl accident, whereas 90Sr in larger part originates from atmospheric nuclear weapons tests. Both of them stay for a long time in the environment because of their relatively long half-live (~ 30 years). Radionuclides can negatively affect people and other living organisms by external irradiation, inhalation, and ingestion, and those in soil take part in all of these three exposure mechanisms. People are normally in the proximity of soil, which causes an external irradiation. Some radionuclides can be released from soil into air, and people can therefore inhale them. Plants absorb radionuclides and incorporate them into the food chain with humans as the final consumers. Internal irradiation inside a body is very hazardous because radionuclides irradiate cells not only with γ radiation but also with α and β particles. In order to carry out the investigations outlined above, gamma-ray spectrometry measurements on 155 soil samples were performed; of these, 138 referred to uncultivated soil and 17 to agricultural soil. Upmost 10 cm layer of soil was sampled. Samples of uncultivated soils were sampled systematically over the whole territory of Croatia. Measurements were carried out using two high purity germanium detectors. The detectors were calibrated using standards containing radionuclides of known activity. Every sample was measured for 24 hours. Measurements resulted in the spectra of gamma rays emitted from samples. By analysing every spectrum "peak by peak", activity concentrations of radionuclides were determined. Since germanium detectors measure only gamma rays, they cannot be used for the detection of radionuclides that are not gamma emitters. However, if a daughter nucleus emits gamma photons and have a half-life considerably shorter than that of its parent, the activity of the latter radionuclide can be determined even if it is not a gamma emitter. This property was used to determine the activity concentrations of some radionuclides that are not gamma emitters. In order to obtain more accurate measurement results, corrections for coincidence summing and for gamma ray attenuation effects were carried out. Coincidence summing correction was performed using the EFFTRAN freely available software. Gamma ray self-attenuation correction generally depends on sample composition, which has not been known for measured samples of soil. Methods for performing gamma ray self-attenuation correction require either a laboratory setup with collimated gamma ray sources or the knowledge of the linear attenuation coefficient of a sample. None of these was applicable to this research. In order to solve this problem, a new method for determining a gamma ray self-attenuation correction was developed. The method combines gamma ray transmission measurements and Monte Carlo simulations. The source of gamma rays for transmission measurements was the sample used for detector calibration. To facilitate Monte Carlo simulations, a software, written in the "R" programming language, was developed. The software uses data from transmission measurements as the input, and calculates correction factors as the output. To calculate the correct activity concentration of a radionuclide, a measured activity concentration is multiplied by the calculated factor. Besides calculating the correction factors, the method also provides the linear attenuation coefficient of a measured sample. On the basis of the measured activity concentrations of radionuclides in soil samples, a general overview of the radioactivity of soil in Croatia has been obtained. Of naturally occurring radionuclides, results for 40K, 238U, 232Th, 226Ra, 210Pb, and 208Tl have been presented, which is usual in studies of that kind. Besides these, results for anthropogenic 137Cs and 90Sr have also been shown. Measured activity concentrations have been presented on the map of Croatia. Each of these maps shows a distribution of the activity concentration of a given radionuclide. Measured activity concentrations have been correlated with geographical, geological, and climatological parameters, as well as with generic soil properties. For easier comparison with radionuclide distributions, some of these parameters have been shown on the map as well. Naturally occurring radionuclides display a correlation with the geological parameters of soil, i.e., with parent material of soil. In the highland and coastal parts of Croatia, where soil parent materials are limestone and dolomite, the activity concentrations of radionuclides from decay series of 238U and 232Th are above average. Within these decay chains, 226Ra shows the best correlation with the distribution of limestone and dolomite. In the Pannonian part of Croatia, where soil parent materials are loess, loams, sand, and clay, the activity concentration of 40K is above average. Significantly higher activity concentrations of 137Cs are found in the highland part of Croatia, while very low concentrations appear in Slavonia. Moreover, a good correlation between 137Cs and the amount of precipitation has been found. Measurements on agricultural soil, where physical and chemical parameters like texture, porosity, water capacity, and cation exchange capacity have been known, indicate that there are other soil parameters which can affect 137Cs retention in soil. On the basis of the measured activities, absorbed doses for external irradiation from naturally occurring radionuclides and 137Cs in soil have been calculated and presented on the map. Very diverse geological, geographical, pedological, and climatological conditions on a relatively small area make Croatia a suitable place for investigating the effects and risks of a possible release of anthropogenic radionuclides into the environment. Measurements of activity concentrations in agricultural soil is the first step towards investigating the transport of radionuclides through the food chain. In addition to the obtained data, observed trends, and drawn conclusions on the radioactivity of soil in Croatia, this work has also resulted in certain general conclusions on the relations between soil properties and the concentrations of radionuclides therein. Systematic and precise measurements of a large number of samples strongly support conclusions. Guidelines for possible future investigations related to this research are also outlined.