We discuss recent experiments that demonstrate how heterointerfaces impact the magnetic properties of heterostructures derived from the "canonical" ferromagnetic semiconductor (Ga,Mn)As. In this material, holes created by the Mn acceptors mediate a ferromagnetic interaction between the Mn ions themselves, and the Curie temperature is determined by a complex interplay between substitutional magnetic ions, interstitial defects and holes. Although as grown epilayers of (Ga,Mn)As typically have Curie temperatures lower than 110 K, post-growth annealing at low temperatures (180 C - 250 C) significantly enhances the ferromagnetic properties, leading to Curie temperatures above 150 K. The first set of experiments examines the effects of capping ferromagnetic Ga1-xMnxAs epilayers with a thin layer of undoped GaAs. We find that the overgrowth of even a few monolayers of GaAs significantly suppresses the enhancement of the ferromagnetism associated with low temperature annealing, suggesting that heterointerfaces have a direct impact on the migration of interstitial defects during post-growth annealing. In the next set of experiments, we demonstrate the first exchange biasing of (Ga,Mn)As by an overgrown antiferromagnet (MnO). Although the exchange bias effect is unambiguous when successful, we also find that the high reactivity between Mn and GaAs affects the ferromagnet/antiferromagnet heterointerface, presenting interesting experimental challenges for the routine achievement of exchange bias in this important spintronic material. This work was carried out in collaboration with K. C. Ku, M. B. Stone, K. F. Eid, P. Schiffer, T. Shih, and C. Palmstrom. Supported by ONR and DARPA.