Chemical elements such as lead circulate in the biogeosphere and through the economy of civilization. It is customary to overview chemical cycles by making simpliﬁed diagrams of principal components, pathways, and places of storage. Such simpliﬁcations are called systems models. On some diagrams symbols are used to show causal relationships. On other diagrams numerical values are placed on the pathways to show at a glance which ﬂows and storages are more important. This chapter uses systems models to overview the principles of heavy metal distribution using the cycle of lead.