When cancer cells spread through lymphatic vessels, the first lymph node they reach is the sentinel lymph node (SLN). Hence, if the SLN is free from metastatic cells, it can be assumed that further lymph nodes will be free of metastatic tumor cells too. If metastatic tumor cells are identified in the SLN, the patient is at risk of developing distant metastatic disease. In that case, further diagnostic and therapeutic steps are needed and the patient’s prognosis gets worse. The SLN can be located at an anatomically unpredictable location. This is likely due to lymphangiogenesis, which alters the normal lymphatic pathway and drains the metastatic cells to another lymph node than the expected anatomically closest one. Therefore, identifying the SLN by an accurate mapping technique is necessary. In this article, the existing techniques for SLN mapping are reviewed, and their use in human and veterinary medicine is compared.