Abstract By systematically investigating the effect of chemical composition on the competitive formation of glasses in various systems, we attempt to address two long-standing scientific puzzles upon metallic glasses, i.e., (i) which composition is the best for forming glasses and glass–matrix composites and (ii) what determines the easy glass-forming composition range in a given alloy system. Our findings have led to the construction of a qualitative microstructure selection map, which is useful for guiding the design of bulkier metallic glasses and glass–matrix composites. In addition, our analysis demonstrates that the classical kinetic treatment of glass formation is insufficient; to analyze glass formation properly, it is necessary to go beyond simple assumptions of single polymorphic solidification during crystallization.