This paper describes the software package Spectrum, which is a modular program that is used to examine the consequences of current trends and future program interventions in reproductive health. It is used to determine the consequences of the HIV/AIDS epidemic, including the number of people living with HIV/AIDS by age and sex, the number of AIDS deaths, and the number of orphans as a result of AIDS, as well as other demographic indicators of interest, such as life expectancy and <5 mortality. The core of Spectrum is a demographic projection model called DemProj, which projects the population by age and sex. Other modules interact with the demographic projection. The HIV/AIDS projections are added to the demographic projections using a module called AIDS Impact Model. This module uses the projection of adult HIV prevalence over time, which is prepared using the Estimation and Projection Package model or the projection workbook. It also requires assumptions about the epidemiology of HIV, including the ratio of female:male prevalence, the distribution of infection by age, the distribution of the time from infection until AIDS death, and the effect of HIV on fertility.