Testicular cancer is one of the most common oncological diseases in young men aged from 14 to 44 years and accounts for approximately 1% of cancer diagnoses worldwide. There is widely varying participation of the immune system between patients arising from the combination of different testicular cancer subtypes within testicular germ cell tumours (i.e. seminoma and non-seminoma) and the normal function of the testis as an immune-privileged organ. In this regard, it is not quite clear if the different immune cells, their subtypes, and respective cytokines and chemokines act as friends or foes when it comes to tumour progression and prognosis. Increasing focus on immune cell presence and their contribution to the tumour microenvironment has stimulated a plethora of studies on other cancer entities; however, our knowledge concerning testis cancer remains limited. Databases have been searched by using keywords ‘testicular germ cell tumor’, ‘immune cells in TGCT’, ‘testis immune privilege’, ‘TIL’, ‘Tumour infiltrating immune cells’, and ‘tumour microenvironment’. There is ample evidence that immune cells (including their subtypes, phenotypical polarisations, and cytokine/chemokine profiles) play an essential role not only in the healthy testis but also in testicular germ cell tumours. Complex interactions between immune cells, other testicular somatic cells, and neoplastic germ cells actively shape a testis-specific tumour microenvironment, which affects tumour growth, metastatic behaviour, and also drug resistance. Intensive research is now being performed to identify immune cell-related risk scores, with the goal of estimating the impact on patients’ prognosis and developing new immune-based therapies. This chapter aims to summarise the up-to-date knowledge of the involvement of immune cells in testicular cancer and includes a brief overview of immune privilege of the testis, development of testicular germ cell tumours, and different methods to assess immune cell presence and activity.