This thesis investigated the ultrasound assessment of the endometrial-myometrial junction (EMJ) and its clinical relevance to the uterine diseases, adenomyosis and endometrial cancer. The inter- and intraobserver variability in the classification of EMJ visualisation using three-dimensional ultrasound was assessed and a high level of agreement was found. Endometrial thickness and parity were found to be significantly associated with the quality of EMJ visualisation. Seven recognised ultrasound features of adenomyosis, including an irregular endometrial-myometrial junction, were investigated for their role in the ultrasound diagnosis of adenomyosis. The other features were an asymmetrically thickened myometrium, parallel shadowing, linear striations, myometrial cysts, hyperechoic lesions and adenomyomas. The presence of any of these features was considered diagnostic of adenomyosis. The inter- and intraobserver variability of ultrasound diagnosis of adenomyosis was also investigated and a good level of agreement was found. This was the case when real-time ultrasound assessments were compared with assessments made from stored uterine volumes, as well as when both assessments were made from stored volumes. Transvaginal ultrasound was used to assess the prevalence of adenomyosis in women attending a general gynaecology clinic in a large prospective observational study. Women were considered to have adenomyosis if one or more ultrasound feature of adenomyosis was found. Using this criterion, the prevalence was estimated to be 20.9% with 7.6% of women being excluded from the data analysis. It was also found that age, gravidity and pelvic endometriosis were all significantly associated with the presence of adenomyosis. Menorrhagia was evaluated in order to assess if it was associated with adenomyosis. Multivariable analysis revealed that while adenomyosis was not significantly associated with menorrhagia when assessed as a binary outcome, when severity of disease was taken into account, there was a significant association. A similar analysis found that adenomyosis was significantly associated with dysmenorrhoea. A second-stage ultrasound test that incorporated assessment of the EMJ was investigated for its use in the diagnosis of endometrial cancer in women presenting with post-menopausal bleeding. It was found to significantly increase the specificity of ultrasound in the diagnosis of endometrial cancer while having a minimal impact on sensitivity.