Ferromagnetic semiconductors grown on semiconductor substrates are being widely investigated as spin injection sources for spintronics applications. Of the many issues critical to the injection efficiency, the band offset plays a central role. In particular, band offsets provide a direct link between microscopic parameters which can be determined theoretically, and macroscopic properties which can be measured experimentally. Moreover, magnetic band offsets can be tuned by methods well known from traditional band-offset engineering, and thus will be important for efforts to optimize injection efficiencies. Here we present first-principles results for the magnetic band offsets in heterostructures consisting of CdCr@sub 2@Se@sub 4@, an n-type ferromagnetic semiconductor with a Curie temperature of 130 K, grown on Si and GaAs substrates. We first use density-functional total-energy methods to explore and identify stable and metastable interface structures, taking into consideration different interface terminations, intermixing, and polar vs. non-polar interfaces. For the thermodynamically favorable interfaces we then apply standard first-principles methods@footnote 1@ for calculating the band offsets. Finally, we present detailed comparisons to recent experiments@footnote 2@ for these heterojunctions. @FootnoteText@ @footnote 1@ A. Franciosi and C. G. Van de Walle, Surf. Sci. Rep. 25, 1 (1996). @footnote 2@ D. Park et al., unpublished. .