Dogs frequently present to first-opinion practice with skin masses, which may require diagnostic work-up including fine needle aspirate and/or biopsy. Cutaneous round cell tumours (CRCTs) (including mast cell tumours, histiocytic neoplasms, plasmacytomas, lymphomas and melanomas) comprise a significant proportion of these masses. Transmissible venereal tumour (TVT) is another CRCT, albeit one less frequently encountered in the UK. Diagnosis of these neoplasms is usually straightforward, but in some cases, these neoplasms can mimic each other (or other tumour types) requiring the pathologist to recommend further tests including immunohistochemistry. Frustratingly, in rare cases, immunohistochemistry results may be equivocal. This presentation provides a brief overview of round cell tumours in dogs as well a review of several case examples.