The theoretical framework associated with Ronald Langacker which has been under development since the mid-1970s and is best represented in his two Foundations of cognitive grammar volumes published in 1987 and 1991 respectively. This is also the most detailed and comprehensive theory of grammar to have been developed within cognitive linguistics, and to date has been the most influential of the cognitive approaches to grammar.

Cognitive Grammar attempts to model the cognitive mechanisms and principles that motivate and licence the formation and use of linguistic units of varying degrees of complexity. Like the conceptual structuring system approach developed by Leonard Talmy and the group of theories known as construction grammars, Langacker argues that grammatical or closed class forms are inherently meaningful. Unlike Talmy, he does not assume that open class forms and closed class forms represent distinct conceptual subsystems.

Instead, Langacker argues that both types of unit belong within a single structured inventory of conventionalized linguistic units which represents knowledge of language in the mind of the speaker, giving rise to a lexicon-grammar continuum. For Langacker, knowledge of language (the mental grammar) is represented in the mind of the speaker as an inventory of symbolic assemblies. The symbolic assembly, which can be simplex or complex, is the basic unit of grammar.

Accordingly, Cognitive Grammar subscribes to the symbolic thesis. It is only once an expression has been used sufficiently frequently and has undergone entrenchment : acquiring the status of a habit or a 'cognitive routine', that it achieves the status of a linguistic unit. From this perspective, a linguistic unit is a symbolic entity that is not built compositionally by the language system but is stored and accessed as a whole.

Furthermore, the linguistic units represented in the speaker's grammar reflect usage conventions. The conventionality of a linguistic unit relates to the idea that linguistic expressions become part of the grammar of a language by virtue of being shared among members of a speech community. Thus conventionality is a matter of degree. For instance, an expression like dog is more conventional (shared by more members of the English-speaking community) than an expression like allophone, which is shared only by a subset of English speakers with specialist knowledge relating to the study of linguistics. The roles of entrenchment and conventionality in this model of grammar emerge from the usage-based thesis. Accordingly, Cognitive Grammar is sometimes referred to as the usage-based model of grammar.

The repository of entrenched symbolic assemblies is conceived in cognitive grammar as a mental inventory.

Yet the contents of this inventory are not stored in a random way. The inventory is structured, and this structure lies in the relationships between symbolic assemblies.

For example, some units form sub-parts of other units which in turn form sub-parts of other units (for example, morphemes make up words and words make up phrases which in turn make up sentences). This set of interlinking and overlapping relationships is conceived as a structured network, and Langacker presents this in terms of a network model. The entities which populate the network of symbolic assemblies are constrained by what Langacker refers to as the content requirement.