Balanced pools of deoxyribonucleoside triphosphates (dNTPs), the building blocks of DNA, and ribonucleoside triphosphates (NTPs), the precursors of RNA, are crucial for a controlled cell proliferation. The dNTPs and NTPs are synthesized de novo via energy-consuming reactions involving low-weight molecules, and through a salvage pathway by recycling (deoxy)ribonucleosides originating from food and degraded DNA and RNA. The enzymes described in this thesis catalyze the first reaction in the salvage biosynthesis of dNTPs and NTPs. The crystal structures of three bacterial thymidine kinases (TKs) are described and the enzymes are investigated as potential targets for antibacterial therapies. TK is a deoxyribonucleoside kinase (dNK) with specificity for thymidine. In addition to the natural substrates, TK can also phosphorylate a number of nucleoside analogs used in antiviral and anticancer therapies. This thesis presents the structures of TKs from three pathogenic microorganisms: Ureaplasma urealyticum (parvum), Bacillus anthracis and Bacillus cereus, and compares them to the human thymidine kinase 1 (hTK1). The bacterial TKs and the hTK1 are structurally very similar and have a highly conserved active site architecture, which may complicate structure-based drug design. However, the different complex structures presented in this work provide information regarding the conformational changes of TK1-like enzymes during the time of reaction. The structure of human uridine-cytidine kinase 1 (UCK1) is also presented. Humans possess two uridine-cytidine kinases, UCK1 and UCK2. The expression pattern of these enzymes is tissue dependent, and despite high sequence as well as structural similarities they possess somewhat diverse substrate specificity. In addition to the natural substrates, uridine and cytidine, UCKs are able to phosphorylate a number of nucleoside analogs. The monomeric structure of UCK comprises four domains: a CORE domain, an NMP-binding domain, a LID domain and a β-hairpin domain, which upon substrate binding undergo dramatic conformational changes. In the structure described in this thesis the enzyme has been trapped in an intermediate conformation between a fully opened and fully closed form, which may represent a sequential mode of substrate binding.