In this doctoral thesis, a comprehensive comparative study of the phytochemical composition and biological effects of selected Thymus species (T. longicaulis C. Presl, T. praecox Opiz subsp. polytrichus (A. Kerner ex Borbás) Jalas, T. pulegioides L., T. serpyllum L. subsp. serpyllum i T. striatus Vahl) from Croatian flora was conducted for the first time. Qualitative and quantitative chemical analyses of bioactive compounds were carried out using various chromatographic (TLC, RP-HPLC-DAD, LC-ESI-MS/MS and GC-MS) and spectrophotometric methods. Rosmarinic acid was found to be the most abundant polyphenolic constituent of all investigated species. Most of the identified flavonoids were from the group of flavones, the largest number belonged to luteolin derivatives, while scutellarin was present with the highest concentration. The investigated Thymus species were significantly different according to the content and composition of essential oils. Thymol and/or carvacrol were found to be the main components of essential oils of T. longicaulis and T. pulegioides, while T. serpyllum subsp. serpyllum was rich in 1,8-cineole. The mountain species T. praecox subsp. polytrichus and T. striatus were characterised by a high percentage of sesquiterpenes. The biological effects of essential oils, plant extracts and phenolic constituents were studied using different in vitro assays. Selected Thymus essential oils inhibited growth of bacterial strains S. aureus, E. faecalis and E. coli. It was found that encapsulation of essential oils in liposomes can maintain or even improve their antibacterial efficiency. The studied plant extracts displayed strong antioxidant properties by acting as free radical scavengers and reducing agents and, additionally, by chelating transition metals and inhibiting lipid peroxidation. The Thymus extracts showed Src tyrosine kinase inhibitory activity and they reduced the production of proinflammatory cytokine IL-6 in Balb/C mice splenocytes. The results of phytochemical and biological investigations suggest that, among Croatian Thymus species, T. longicaulis has the greatest potential for pharmaceutical usage comparable to that of pharmacopoeial species T. vulgaris.