Cancer poses a major public health problem in the Republic of Croatia and it is the second leading cause of death, after cardiovascular disease. Solid tumours account for a large percentage of cancer mortality and morbidity, and an increased incidence has been observed as the population ages. Therapy of solid tumours is an extremely complex subject of research, and recent findings allow us to find new targets and procedures for therapy. Recent research has shown that solid tumours regulate the immune response and prevent an immune attack in the tumour microenvironment, in a process described as cancer immune escape. One of the key components in the tumour escape of the immune system is the PD-1 molecule found on the surface of activated T lymphocytes, which interacts with PD-L1 and PD-L2. Tumour cells, but also several other cell types, express PD-L1 on their surface and thus eliminate the attack of cytotoxic T lymphocytes. By blocking this inhibition, exhausted T lymphocytes that inhabit the tumour microenvironment regain the effector phenotype and commence a renewed attack on neoplastic cells. Immunotherapy based on the blockade of the PD-1/PD-L1 signalling axis has shown significant potential in clinical trials. In the Republic of Croatia, this type of immunotherapy has entered clinical practice in the case of several solid tumours, primarily melanoma, triple-negative breast cancer, non-small cell lung cancer, bladder cancer and renal cell carcinoma. In this thesis, the details related to the effect of the PD-1/PD-L1 axis on the immunoediting of the tumour and the clinical implications of its blockade are discussed.