From the early 80s of the past century, clinical and epidemiological studies have increasingly pointed to an elevated breast, endometrial and ovarian cancer risk from high circulating oestrogen levels. Oestrogen deficiency as a cancer risk factor emerged first in 2007 in a study on Hungarian oral cancer cases based on the gender and age-related differences in tumour prevalence. Critical re-evaluation of the contradictory results of hormone replacement therapy yielded a complete conversion that oestrogen deficiency may confer cancer initiation. This book examines the deleterious interactions between oestrogen deficiency and further hormonal disorders which illuminate the apparently controversial associations of obesity, light exposure and alcohol intake with breast cancer risk.