The situation as regards teachers of sex-instruction is very different from that of all other subjects concerning which young people should be taught. We cannot safely plan the teaching regarding sex until we know the teacher. This will be evident, I think, after some general considerations concerning selection of teachers and some discussion of problems such as the first teacher, teachers for classes, and some undesirable teachers. The general rule should be, first, find the safe and sure teacher and, second, select the facts and plan the lessons that the chosen teacher may give effectively. [Sidenote: Teachers of same sex for children.] So far as young children are concerned, the needed instruction is so general in character that the sex of the competent teacher is of little importance, but the information that ought to prepare for and guide through the mazes of adolescent youth should come to young people from teachers of the same sex. If exceptions must be made rather than omit instruction altogether, some very mature women may safely guide both boys and girls through adolescence; but men, even physicians, should not undertake instruction of adolescent young women, unless parents and other mature people are present to help with attitude. That women may well instruct boys I know, because the most impressive sex lecture I ever heard was given by the late Dr. Mary Wood-Allen to the boys of the freshman class when I was a college student. But note that I have said "some very mature women." The fact is that I fear danger for some boys if they are frankly instructed by attractive young women who are only ten to fifteen years older than their pupils. Hence, I urge great caution if there must be any exceptions to the general rule that teachers and pupils should be of the same sex. [Sidenote: Coeducational classes.]