This chapter focuses on crowding in the household, looking at its effects both as objectively and subjectively measured, and provides a stress model of crowding. This model suggests that the more households are crowded, the more stressful the situation. The effects of human crowding loom as an ever larger and more pressing issue. World population stands at over five and a half billion people. There are several disparate and relatively long traditions of crowding research. Each tradition approaches the subject of crowding somewhat differently. One tradition of research concentrates on animals other than humans. Under crowded conditions, some species of animals have been observed to undergo population crashes, whereby their numbers are reduced to a level that permits the remaining population to sustain itself. Territorial behavior appears to be biologically-driven, and there is no credible evidence that there is any such counterpart among humans. The chapter also presents an overview of the key concepts discussed in this book.