This article reviews proposed technical approaches for the manufacture and use of alternatives to Portland Cement Clinker as the main reactive binder component for ordinary concrete construction in nonspecialty applications, while giving lower net global CO2 emissions in use. A critical analysis, taking into account a wide range of technical considerations, suggests that, with the exception of alkali-activated systems, (treated in a separate paper in this issue,) there are only four classes of alternative clinker system that deserve serious attention with respect to global reductions in concrete-related CO2 emissions: (A) Reactive Belite-rich Portland cement (RBPC) clinkers (B) Belite-Ye'elimite-Ferrite (BYF) clinkers (C) Carbonatable Calcium Silicate clinkers (CCSC) (D) Magnesium oxides derived from magnesium silicates (MOMS) A and B are "hydraulic" clinkers, (i.e. clinkers which harden by reaction with water,) C is a "carbonatable" clinker, (i.e. one which hardens by reaction with CO2 gas) and D can fall into both categories.