Semantic features or primes, like phonetic and morpho-syntactic features, are usually considered as basic elements from which the structure of linguistic expressions is built up. Only a small set of semantic features seems to be uncontroversial, however. In this article, semantic primes are considered as basic elements of Semantic Form, the interface level between linguistic expressions and the full range of mental structures representing the content to be expressed. Semantic primes are not just features, but elements of a functor-argument-structure, on which the internal organization of lexical items and their combinatorial properties, including their Thematic Roles, is based. Three types of semantic primes are distinguished: Systematic elements, that are related to morpho-syntactic conditions like tense or causativity; idiosyncratic features, not corresponding to grammatical distinctions, but likely to manifest primitive conceptual conditions like color or taste; and a large range of elements called dossiers, which correspond to hybrid mental configurations, integrating varying modalities, but providing unified conceptual entities. (Idiosyncratic features and dossiers together account for what is sometimes called distinguishers or completers.) A restricted subsystem of semantic primes can reasonably be assumed to be directly fixed by Universal Grammar, while the majority of semantic primes is presumably due to general principles of mental organization and triggered by experience.