The doctoral thesis is based on eight papers published in peer-reviewed journals and a review of the literature. The papers are published between 1997 and 2013 in cooperation with Sankt Elisabeth Hospital, Herlev Hospital, Glostrup Hospital, Rigshospitalet, Hvidovre Hospital, Amager Hospital, Copenhagen Trial Unit, and Institute of Preventive Medicine, Copenhagen. Groin injuries in sport are very common and in football they are among the most common and most time-consuming injuries. These injuries are treated very differently around the world. There is no consensus in the literature regarding definitions, examination methods, diagnosis or treatment and in general the level of evidence is very low. There is a need for identification of the painful anatomical structures, how to examine them and how to define clinical entities to develop effective treatment and prevention. The aim of these studies were: - To review the literature to create an overview of the ideas and the knowledge in order to plan future studies in this field. - Develop and test clinical examination techniques of the relevant tendons and muscles in the region. - Since no evidence-based diagnosis exist; to develop a set of clinical entities to identify the different groups of patients. - To test the effect of a dedicated exercise program developed for treatment of long-standing adductor-related groin pain in athletes in a randomised clinical trial comparing it to the treatment modalities used at that time. - To examine the long-term effect of the above mentioned training program for treatment of long-standing adductor-related groin pain. - To develop a training program for prevention of groin injuries in soccer and test it in a randomised clinical trial. - To describe the occurrence and presentation in clinical entities of groin injuries in male football and to examine the characteristics of these injuries. - Evaluate if radiological signs of femuro-acetabular impingement (FAI) or dysplasia affect the clinical outcome of treatment of long-standing adductor-related groin pain, initially and at 8-12 year follow-up. The main findings of the eight papers were: - No randomised trials existed in this area; there was no consensus in the literature and the majority of the literature was Level 4 and 5. From the existing literature and the author' experience an injury mechanism was suggested and the term ''adductor-related groin injury'' was suggested. - A well-defined clinical examination of the adductor-, iliopsoas, and abdominal muscles and the symphysis joint for pain, strength, and flexibility was reproducible with only limited intra- and inter-observer variation. - By utilising a well-defined classification long-standing groin injuries could be classified with a system of clinical entities.