PIP: The scientific foundations and most notable features of modern natural family planning (NFP) methods are described. NFP techniques require observation of the signs and symptoms occurring during the fertile and infertile phases of the menstrual cycle. Abstinence during the fertile period is implied when NFP is used to avoid pregnancy. Modern NFP methods are associated with success rates similar to those of oral contraceptives, the IUD, and condoms. NFP methods are based on such phenomena as the limited survival time of the ovum and sperm, the occurrence of ovulation only once per cycle, the ability to diagnose ovulation using simple means, and the ability of human beings to delay sexual satisfaction. Among the advantages of NFP methods are low cost, brevity of the required training time, increased knowledge of the body and capacity for self control, shared responsibility by both partners, and moral and religious acceptability. The Billings and symptothermal methods have shown average use-effectiveness rates ranging from 71.5 to 89.5% and from 83.4 to 97.8%, respectively. All studies of symptothermal methods conducted since 1985 have shown Pearl indexes under 5. The symptothermal method is based on observation of the duration of previous cycles, cervical mucus, basal body temperature, and optionally on palpation of the cervix and other symptoms such as breast swelling, acne, intermenstrual bleeding, and mood changes. Use of the symptothermal method requires special graph paper for recording the temperature and other symptoms. The beginning of the fertile phase is recognized by subtracting 19 from the shortest cycle in the past year, or by the appearance of cervical changes or changes in the mucus.