The transition metal (TM) chalcogenides of the form TMX₂ (X = S or Se) have been studied for decades due to their interesting electronic and magnetic properties such as metamagnetism and metal–insulator transitions. In particular, the Co_1−xFe_xS₂ alloys were the subject of investigation in the 1970s due to general interest in itinerant ferromagnetism. In recent years (2000–present) it has been shown, both by electronic structure calculations and detailed experimental investigations, that Co_1−xFe_xS₂ is a model system for the investigation of highly spin polarized ferromagnetism. The radically different electronic properties of the two endpoint compounds (CoS₂ is a narrow bandwidth ferromagnetic metal, while FeS₂ is a diamagnetic semiconductor), in a system forming a substitutional solid solution allows for composition control of the Fermi level relative to the spin split bands, and therefore composition-controlled conduction electron spin polarization. In essence, the recent work has shown that the concept of 'band engineering' can be applied to half-metallic ferromagnets and that high spin polarization can be deliberately engineered. Experiments reveal tunability in both sign and magnitude of the spin polarization at the Fermi level, with maximum values obtained to date of 85% at low temperatures. In this paper we review the properties of Co_1−xFe_xS₂ alloys, with an emphasis on properties of relevance to half-metallicity. Crystal structure, electronic structure, synthesis, magnetic properties, transport properties, direct probes of the spin polarization, and measurements of the total density of states at the Fermi level are all discussed. We conclude with a discussion of the factors that influence, or even limit, the spin polarization, along with a discussion of opportunities and problems for future investigation, particularly with regard to fundamental studies of spintronic devices.