In the past two decades mid-IR nonlinear optical crystals have grown from scientific curiosities to practical robust materials generating efficient, multi-watt output in the 3-12μm spectral range. Nonetheless, improved NLO crystals are critical for further advancing mid-IR laser development. In particular, mid-IR materials are needed which: 1) efficiently convert cw pump sources; 2) can be pumped with 1-micron (Nd and Yb) lasers; and 3) offer improved performance in the 8-12 micron range and beyond. For such applications a high nonlinear coefficient is desirable, but this is less important than more practical properties such as low absorption loss, high laser damage threshold, low thermal lensing, and non-critical phase matching (NCPM). To meet these demands, the search for improved mid-infrared NLO crystals is proceeding on three fronts. First, work continues to reduce losses in the best existing materials such as ZnGeP2, AgGaSe2, and CdGeAs2. Secondly, new compounds such as AgGaGeS2 and AgGaGe5Se12 are being evaluated. Thirdly, and most promising, is the work on all-epitaxial processing of orientation-patterned gallium arsenide (OPGaAs) and other binary quasi-phase-matched semiconductors such as ZnSe, GaP, and GaN. This review paper will discuss recent advances on all of these fronts.