This pamphlet is an exposition of the author's method of determining the so-called “safe period.” Although the existence of a period of physiological sterility in the menstrual cycle is probable, its determination by the method suggested, which utilizes at least four variable factors (regularity of the menstrual cycle, exact time of ovulation, and definite viability periods for both sperm and ovum), treating them as relatively constant, is hardly to be recommended to anyone to whom pregnancy might spell disaster.