Research work within the project "Radiochemistry and Radioecology" was focused on various fields of analytical work involving the use of nuclear and nuclear-related techniques. The development of methods for determination of natural and man-made radionuclides using alpha and gamma spectrometry, beta counting and liquid scintillation techniques was an important part of project. We investigated main critical steps of such procedures: dissolution of samples, leaching or preconcentration from a complex matrix, chemical and radiochemical isolation of the analyte radionuclide from the sample, and preparation of a thin, virtually weightless source for high resolution alpha spectrometry. Methods for determination of uranium, thorium, polonium, plutonium and americium radioisotopes by alpha spectrometry were developed. For determination of the long lived Np-237 isotope a method was developed based on combination of alpha and gamma spectrometry and preseparation radiochemical neutron activation analysis. The liquid scintillation counting were used for determination of Ra-226 in drinking and mineral water. Both, alpha spectrometry and liquid scintillation counting were used for determination of Ra-226, Th-230 and Pa-231 in ecological samples. For determination of strontium radioisotopes, Pb-210 and Po-210 crown ethers were used as extraction media. Neutron activation analysis (NAA) offers important advantages for the analysis of trace and minor elements, especially regarding analytical quality control, due to its inherent characteristics such as matrix independence, virtual freedom from blank problems and specificity, as well as its totally independent nature as a nuclear-based method, rather than on electronically-based phenomena as in other spectroscopic methods. Particular attention was paid to development of methods for determination of trace uranium, thorium, arsenic, iodine, selenium, vanadium and manganese in ecological and biological samples by radiochemical neuron activation analysis (RNAA). In RNAA an important source of error is the uncertainty of the chemical yield. Therefore radioisotopic tracers were used for each sample aliquot. In addition, methods for determination of various chemical species of arsenic and selenium were developed. Regarding the non-destructive or instrumental mode of NAA (INAA), the most attractive and rapidly developing form is the technique of k0-standardisation. This method is very relevant and applicable for environmental studies. It enables multielement analysis of a range of sample types. We estimated the overall accuracy of the k0 method by evaluating all possible errors which can arise in the various parameters required, with consequent quantification of the most critical ones. Investigation of the dependence of the paramneter f (thermal-to-epithermal fluence rate ratio) and the mass of Cd used for its determination by the recommended method was made. We established the distribution of neutron fluence rate in steady-state operation mode of the TRIGA reactor in the vicinity of irradiation channel 20, and the axial and radial neutron fluence rate gradients within various organic and inorganic sample matrices, and the self-shielding factor for thermal neutrons. Nuclear parameters (T1/2, Q0, Er and k0) were critically reviewed and adopted for analytical purposes. The emphasis of radioecological studies was focused on investigation of the cycling and intake of heavy metals and radionuclides and of dose assessment. Lichens and mosses were used as biomonitors.. A large biomonitor database was created to be used in identification of emission sources and in estimating their influence on the environment and public health.