Erratic, "unscheduled", vaginal bleeding continues to be the greatest disadvantage associated with widespread use of long-acting, progestogen-only methods of contraception. As a consequence, it is also the main reason for premature discontinuation of use of these methods in most cultures. From other perspectives, these methods have high acceptability, very high contraceptive efficacy and a range of valuable, added, non-contraceptive health benefits. There has been widespread awareness of the variability of these vaginal bleeding patterns associated with long-acting methods for several decades and much research has been invested into studying their patterns and implications. Considerable research has also been directed towards trying to understand the underlying mechanisms responsible for the unpredictable bleeding. Much has been clarified about the multiple mechanisms contributing to the appearance of superficial, thin-walled fragile vessels within the endometrium of many of those women with troublesome bleeding, but there is still little understanding of why some women develop these vessels and others have no fragile vessels (and may therefore develop amenorrhea). We now have several medical approaches to reliably stopping a prolonged episode of troublesome bleeding, but no good therapy to produce long-lasting relief from recurrence of erratic bleeding in predisposed women. Future understanding of the variability in individual endometrial responses in different women may be a key to solving this frustrating symptom.