Natural family planning (NFP) is based on the knowledge *largely nonexistent) of a women as to whether she is in her fertile period or not. In contrast to the calendar method, the Billings method consists of observing bodily functions, whereby women learn about the fertile and infertile period during the menstrual cycle. This method is very safe as long as the woman has been instructed thoroughly. The Pearl index (the number of pregnancies/100 woman years) can be 1. In a Swedish province, 7/1000 population used this method and had an abortion rate of .5/1000, a fact contradicting the allegation of mass abortion as a result of the method. Only well-trained NFP instructors can teach women, and at the University of Umea such training has been available for some years. The biological basis of the Billings method rests on the fact that every release of an egg is preceded by a ripening process of a follicle in the ovary. This follicle secretes increasing amounts of estrogen which stimulates the cervix to produce secretions for the sperm. Right before ovulation, the follicle reduces estrogen production and noradrenaline takes over, stimulating the peak-day secretion (P-secretion) for further selection of sperm. Ovulation usually occurs on the peak day, which is the day of maximum fertility and the last day of mucous symptoms. For 3 days after peak day until menstruation, the risk of becoming pregnant diminishes successively until it is as low as after sterilization. The instructor is trained to recognize and overcome certain factors that make it more difficult to identify the mucous symptoms, such as the previous use of oral contraceptives, certain illnesses, drugs, and life styles. NFP can also be used for attaining pregnancy by identifying the peak day; women with premenstrual syndrome can calculate when their symptoms start, and sportswomen can predict the time of their menstruation. NFP is fascinating when it is compared to other methods because of its human dimension, and there is still a great deal to be learned about it.