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.