The purpose of this paper is to study the relationship between maps with infinite essential category weight and phantom maps (there is a brief summary of the main results on essential category weight in the appendix to this paper). It is not hard to see that any map with E(f) = oo is a phantom map. We give examples to show that the converse is not always true: there are phantom maps f with E(f) = 1. We also show that if ¦¸X is homotopy equivalent to a finite dimensional CW complex then every phantom map f: XiaY has E(f) =iÞ. We are able to adapt many of the results of the theory of phantom maps to give us results about maps with E(f) = iÞ. Finally, we use the connections between essential category weight and phantom maps to answer a question (asked by McGibbon) about phantom maps.