Abstract: The testicles are the second most common location for tumours in the male dog. Mostly, testicular tumours are primary tumours (leydigoma, sertolinoma and seminoma), which are benign in 80% of cases. Metastases occur late in the condition and disseminate, usually via the lymphatics, to the lymph nodes although lung metastases by the haematogenous route are possible. Paraneoplastic syndromes sometimes occur because of the functional ability of sertolinomas (hyperoestrogenism mainly) and leydigomas (hyperandrogenism more rarely). The diagnosis is mainly by clinical examination. It is complemented by medical imaging examinations and evaluation of the secreting capacity of the tumour in order to assess the spread of the tumour and to make a prognosis.