Every year, a large number of surplus pets are relinquished to animal shelters. This can result in problems, including distress in the animals and the owner, and may stretch limited resources. To lower the number of surplus pets we need to know the reasons for relinquishment. The objective of this study was to identify the reasons for relinquishment of dogs and cats relinquished to a large Danish shelter, and to investigate whether these are related to the animal (e.g. behavioral problems), or the owner (e.g. illness), and developments over time.