How to achieve a reliable diagnosis – even in complex clinical cases. For such challenges, veterinary surgeons must know about all the possible pathophysiological causes of a particular clinical problem, be able to evaluate them in individual cases and exclude anything which is not relevant using diagnostic pathways. The 'rules out' describe this process of exclusion for all aspects of internal medicine in the cat and dog, and ultimately lead to a reliable and secure diagnosis. They form, therefore, an important basis for everyday clinical work and provide a practical guideline for the systematic problem-oriented processing of patients. For students, these 'rules out' are a prerequisite for understanding veterinary internal medicine. For experienced clinicians, they are a helpful diagnostic guide especially when dealing with complex cases and so can help in solving particularly 'knotty' problems.