Linguistic expressions are instrumental in categorizing the world. Prototype semantics holds that categorization is centered on best exemplars, or prototypes, with other potential members of the categories evaluated in accordance with their similarity to it. Prototype effects have been documented in language acquisition and learning, reaction times, priming effects, saliency in listing tasks, etc. The debated issues include what the prototype really is and whether it changes with context. An important extension of categorization research concerns cross-linguistic comparison: what is universal and what is language specific in linguistic categorization and to what extent the emerging categories can be accounted for by reference to prototypes.