Dairy fermentations are constantly threatened by the presence of bacteriophages, which can lead to acidification failures and a lower quality of final products. Phage infections of Streptococcus thermophilus is a paramount issue, due to the economic importance of this bacterium for cheese and yoghurt production worldwide. Advances in genomics and molecular biology open new perspectives for developing effective strategies to prevent phage infections. This PhD thesis focuses on investigating biodiversity of S. thermophilus phages and understanding their interactions with a host. Phages infecting S. thermophilus have been considered a homologous group. In the presented study, two novel variants of S. thermophilus phages were discovered. The studied phages possessed genomic and morphological features atypical for dairy streptococcal phages. They seemed to evolve by genetic recombination between phages infecting different species from dairy and non-dairy environment. A detected diversity within the S. thermophilus phages led to proposing a new classification of this group. Phage monitoring is crucial to control phage outbreaks during dairy fermentations. The novel types of S. thermophilus phages could not be detected with PCR methods that have been applied to identify dairy streptococcal phages. Therefore, an improved multiplex PCR assay was developed, enabling rapid detection and classification of all known types of these phages. One of the major objectives of the presented PhD study was to identify phage receptors of S. thermophilus. To that end, spontaneous phage-resistant mutants were generated. Based on the genome sequencing data, phage resistance in putative receptor mutants was attributed to mutations in genes encoding biosynthesis of glycans. As visualized using super-resolution structured illumination microscopy (SR-SIM), phages were localized either at the septum or along the cells. The results of phenotypic and biochemical assays indicated that phage adsorption to S. thermophilus is presumably mediated by glycans associated with bacterial cell surface. In the final investigation, comparative genomics was applied to explore biodiversity of S. thermophilus phages and predict phage-host relationships. A comparative analysis was performed for 115 phage genomes, of which 55 were sequenced in the presented study and 60 were available in the public database. Based on the results, four groups of S. thermophilus phages were defined. Subsequently, it was confirmed that phage antireceptor plays a role in host recognition. Finally, a correlation between genes encoding receptor and antireceptor was observed. The knowledge obtained in the presented PhD thesis will contribute to developing strategies to prevent phage outbreaks in the dairy industry. Updated classification of S. thermophilus phages allows improving the phage monitoring. The identity of phage receptors gives opportunity to develop phage-resistant mutants with broad protective range. The observed correlation between the genotype of the cell surface receptor and the phage antireceptor will aid in developing efficient approaches to combat phage infections in dairy plants.