Hormone replacement therapy consists of two hormones: estrogen and progestogen.  Women who have an estrogen deficiency will notice several changes with the onset of menopause, including hot flashes, vaginal changes, loss of interest in sex, and a general change in their mental outlook such as poor memory, gloominess, and irritability.  Hormone replacement therapy is designed to alleviate these symptoms, and for the majority of women it works quite well with very few side effects.