The overall high prevalence of pelvic organ prolapse (POP) and related problems indicates a need for identification of risk factors, prevention and treatment strategies. The aetiology of POP is considered to be multifactorial. Only a minority of the many proposed risk factors for POP can easily be prevented. Treatment of POP is surgery, use of a pessary or pelvic floor muscle training (PFMT). Today there is a lack of evidence for the effect of PFMT in reducing and preventing POP, and there is scant knowledge of possible changes in morphology and function of the pelvic floor muscles (PFM) following PFMT. Proposed changes in the pelvic floor include increased muscle volume, elevated positions of the pelvic organs and narrowing of the levator hiatus. Use of responsive, reliable and valid outcome measures is mandatory for evaluation of these proposed effects. 3D/ 4D perineal ultrasound is a new promising assessment tool. However, its reliability and validity must first be tested. The overall aim of this thesis was to establish intra tester reliability of the ultrasound measurements, identify risk factors for POP, evaluate the effect of PFMT in prevention and treatment of women with POP and to assess morphological changes after PFMT.