Cancer is a leading cause of death in older companion dogs and cats, with approximately 1 in 4 dogs developing cancer during their lifetime. Welfare considerations in companion animal oncology are complex: treatment may extend life while causing treatment-related suffering (nausea, fatigue, immunosuppression), or may prioritise quality over quantity of remaining life. Validated quality-of-life assessment tools (VetMetrica, FETCH, Canine Quality of Life Scale) provide structured frameworks for treatment decision-making. Chemotherapy in veterinary medicine typically uses lower doses than human oncology, producing less severe side effects while maintaining anti-tumour activity. Palliative and hospice care focuses on pain management, nutritional support, and maintaining normal activity. Research demonstrates owners frequently overestimate remaining lifespan and underestimate treatment burden; honest communication is a welfare-positive veterinary responsibility. Financial constraints significantly affect treatment access, raising equity dimensions in companion animal welfare.