As a result of the shortage in the availability of suitable steel scrap, trace elements are unintentionally added to ductile iron from the scrap available for melting. The effect of some of these trace elements on graphite shape, the resulting microstructure, and the dimensional behavior of the cast component are not well understood. The lack of control of these trace elements leads to excessive scrap as well as additional heat treatment costs, especially when ferritic or fully pearlitic microstructures are required. This work focuses on the effect of one element, copper, that occurs as a trace element or is often deliberately added when pearlitic microstructures are desired. Ductile iron samples with copper levels ranging from 0 to 0.8 wt. % were investigated. Gleeble dilatometry was used to characterize phase transformations and microstructure development. The diffusion coefficient of carbon in ferrite in the presence of copper and silicon was measured using multicomponent solid-solid diffusion experiments. Copper appears to have little or no effect on the diffusion coefficient of carbon in ferrite. Interrupted solidification experiments are used to explain solidification and segregation in ductile iron, and a revised model of ductile iron solidification is presented. It is shown that the segregation of copper during solidification is key to the pearlite promoting effect of copper and is related to the decrease in the driving force for the diffusion of carbon through the ferrite shell.