While it's desirable to have all of the parts of an automobile in good working order, one would hardly want all of the components to be active at the same time. Nor would one want any one of the components, such as the horn, to be active all of the time. Too much of a good thing is not a good thing. This is also true of one's genome. There are times for genes to be active and times for them to be inactive. The ingenious mechanisms that have evolved to regulate the activity and inactivity of the genes are the subject of this chapter.