Trend theory has been designated as an effort to explain heroin use trends. Its purpose is to answer the epidemiological question of why these individuals in this time and place experience an increase in heroin use? Trend theory integrates traditional epidemiological indicators and contextual material. Two functions representing trend theory's basic assertion are presented and used to explain the basis for the theory. First, trends are a function of context and second, the "iterative map" shows the dynamic nature of trends. The two functions show how patterns of use rise and fall in interlinked contexts and how these patterns and contexts change over time. The article takes these two functional equations and applies them to two detailed cases where within the populations a heroin epidemic occurred involving a historical situation. The first case involves contemporary suburban white youth and the second, involves African-Americans in the early to mid-1960's. It was hypothesized that sudden historical stress has psychological consequences that induce frustration, anger, and depression. In examining both cases, changes were seen in population and in the delivery system. The populations were in a situation of rapid historical change, opening an extensive gap between expectations and reality. In addition, heroin production and/or distribution changed in sudden and dramatic ways. The message of trend theory is that policies come and go, but open marginality and organizational crises will stay. For trend theory, it makes sense to treat policy as part of changing environments as opposed to a separate item in the equation. The epidemiological observation was that heroin epidemics are never randomly distributed in a society. References