[Truncated abstract] A diagnosis of breast cancer is a life-changing event for any woman. For young women and their families it can be devastating. Women aged less than 45 years make up 20% of new cases of breast cancer diagnosed annually in Australia. With the trend for women to delay pregnancy, young women diagnosed with breast cancer may want at least the option to become pregnant after diagnosis and treatment but little is known about how pregnancy affects breast cancer or how breast cancer affects pregnancy. The aims of this thesis were to investigate how concurrent and subsequent pregnancy affects the development and outcomes of breast cancer and how breast cancer affects a concurrent or subsequent pregnancy. This study describes two groups of women identified from the entire Western Australian population less than 45 years of age when diagnosed with: 1. Gestational breast cancer, defined as breast cancer diagnosed while a woman is pregnant or in the first twelve months after completion of a pregnancy; and 2. Breast cancer who subsequently conceive. This study focused on three main areas; patterns of care and outcomes for women diagnosed with gestational breast cancer and those women diagnosed with breast cancer who subsequently conceived; the imaging and pathological characteristics of gestational breast cancer; and lastly the psychosocial issues associated with gestational breast cancer. ... This result was statistically significant. In an age and staged matched case control study lymph node negativity did not purvey a survival advantage for women diagnosed with gestational breast cancer as it did for the non- gestational breast cancer controls. Women diagnosed with breast cancer who have good prognosis tumours need not necessarily wait two years to become pregnant. In an age matched case control study women diagnosed with gestational breast cancer were more likely to have extensive insitu carcinoma, higher mitotic rates and tumours with medullary like features than their age matched controls. In a Cox's proportional hazards regression model which included pathological characteristics, there was no significant difference in survival for women diagnosed with gestational breast cancer were compared to women diagnosed with non-gestational breast cancers. The psychosocial issues for women diagnosed with gestational breast cancer are similar to other young women diagnosed with breast cancer but the effect on the 9 lives of women dealing with pregnancy and breast cancer simultaneously was much greater. The issues of breast cancer and pregnancy are complex at both a physical and psychological level. Much more research is needed to understand the mechanisms of how pregnancy affects breast cancer and its spread. Women who are pregnant when diagnosed with breast cancer or who consider pregnancy after their diagnosis need unbiased support from those around them. Survival is important but other survivorship issues may be just as important. To translate these findings into clinical practice and offer directions for future research eleven recommendations are proposed.