The book's first chapters cover physical changes which range from testicular growth, including advice on testicular self-examination and reassurance that it is perfectly normal for one testicle to grow larger and hang lower than the other ("with time, the other testicle will catch up, and they will both be about the same size when you are fully developed," 12); and further provide factually reassuring information about penis size, hair in new places, growth spurts, pimples, perspiration, voice changes, erections, ejaculation, orgasm, wet dreams, and masturbation.
Many people masturbate, including children, teenagers, married and unmarried adults, and elderly folks. They all do it for one reason: It feels good. (43)The bulk of the book addresses the topic that probably interests boys the most: sex; i.e., being sexual, having sex, being sure you are ready, not pressuring others, homosexuality (it's not a choice, 72), birth control, STDs, and STD preventions.
It is not harmful, will not make you run out of sperm, and it will not "ruin you" for normal sexual relations. In fact, masturbation probably helps prepare you for sex with another person. By exploring your own body, you have a chance to learn what feels best to you. (46-47)
One chapter addresses typical "what's normal" concerns about crooked penis shape, masturbating with other boys (it's normal sex play for boys, 115), and more. Another chapter responds to "what if" questions ranging from "what if I have an erection when I'm giving a report in front of my class" to "my girlfriend and I had sex, and she got pregnant. I think she should get an abortion, but what if she doesn't want to?" (it's her choice, 125).
A final chapter is a collection of advice from adult men thinking back on adolescence and what they wish they'd done differently, from being able to accept yourself more to knowing more about the changes of adolescence in advance.
What's Going on Down There? also devotes an entire chapter to how girls are affected by puberty, providing a detailed description of girls’ external and internal sex organs and how they change during puberty, as well as menstruation (does it hurt, and why does it make girls cranky?).