Population momentum is extremely important in understanding the dynamics of population growth and its consequences for population policy. Although momentum is discussed in almost every introductory population course, its demographic components and their policy implications are easily misunderstood. Population momentum refers to the fact that the future growth of any population will be influenced by its present age distribution. Such momentum is the reason why replacement-level fertility does not immediately translate into zero population growth. Discussions of future population growth routinely mention that even if world fertility falls to replacement level, momentum will cause population to continue to increase for some time. Fertility and death rates need to be understood in order to understand population momentum. The effects of positive and negative momentum upon populations at replacement-level fertility are discussed.