Health Impact Assessment (HIA) is a set of procedures and methods that provides information to decision-makers—both in the health sector and in other fields—about the potential impact an intervention may have on the health of the population. HI is an underutilized tool in Spain, unlike in other European countries. This article presents the methodology used in HI and describes the stages and key elements for its implementation. The Apheis project, in which five Spanish cities have participated, serves as an example to demonstrate the usefulness of HI in decision-making and for improving public health plans and programs.