This paper describes the contributions made by Władysław (Władek) Walukiewicz and his colleagues to the field of ferromagnetic semiconductor (FMS) alloys, such as (Ga,Mn)As. We focus on two key accomplishments. First, this team has predicted the formation of Mn interstitials in these materials, which have a profound effect on ferromagnetism in semiconductors. Additionally, identifying the conditions at which interstitials form has provided grounds for optimizing their ferromagnetic properties. Second, by applying the approach of band anticrossing to ferromagnetic semiconductors, this team has mapped out the properties of an Mn-derived impurity band in these materials. This is of particular importance in the field, because holes, which reside in the Mn-derived impurity band, are the very mechanism responsible for ferromagnetic order in FMSs. We discuss the effect that these accomplishments have on our understanding of FMSs and how they have contributed to progress in this area. We then describe the pathways that these achievements have opened up toward further progress in both basic and applied fronts of ferromagnetism in semiconducting systems; and we present our perspective on where additional work along the lines initiated by Władek Walukiewicz should be extended to further benefit this field.