Pericardial effusion with cardiac tamponade and right congestive heart failure and / or collapse is the most common presentation of pericardial disease in dogs. The most common causes of pericardial effusion are tumours (mainly aortic-base chemodectomas and thyroid carcinomas, hemangiosarcoma and mesothelioma) or idiopathic pericarditis. However, there are other less common causes and these should always be taken into account. After draining the effusion and solving the tamponade, the clinician should wonder about the next steps. What tests should I use to find out the underlying cause? Which are the less common causes that could be occurring? What treatment should I use after the corresponding investigation in each clinical situation? In this presentation, and based on current scientific evidence and the author's experience, the benefits and problems of different diagnostic tests and treatments to be performed in the most common clinical situations will be described.