Philosophers interested in historical explanation have tended to focus on causal explanations of particular events, states of affairs, or observable traces. Yet researchers working in fields ranging from paleontology and evolutionary biology to climate science and economics seek to document and explain the occurrence of historical, population-level trends, where a trend is a persistent, directional change in some state variable. Examples of such trends include everything from evolutionary size increase (Cope’s rule), to changes of gene frequencies in an evolving population, to global warming. This chapter explores the idea that explanations of historical trends are typically causal explanations. Woodward’s interventionist theory treats causation as a relation between variables and so lends itself readily to the idea that trends can be causes and effects. A small extension of Woodward’s theory can help illuminate cases in which scientists talk of one trend as being the cause of another. However, paleontologists often explain trends by claiming that they are passive, that is, that they involve a random walk away from a fixed boundary in the state space. This type of explanation of an historical trend does not invoke any causes in the interventionist sense, which suggests that some explanations of historical trends might not be causal explanations.