Definition of the copula as a discrete grammatical category is problematic. It is the semantically unmarked copulas (simple equivalents of the English verb ‘to be’) which deserve most attention in a comparison of the Romance languages; they have a typically suppletive historical morphology and are often the result of the grammaticalization of full lexical verbs, the point at which true unmarked copular status is achieved being sometimes difficult to identify. The unmarked copulas of the Ibero-Romance languages (Spanish, Portuguese, and Catalan) have the most complex distribution and have proved the most difficult to account for synchronically. The situation in the early 21st century is the consequence of a progressive encroachment of the reflexes of Latin stare and other verbs on the functions of Latin esse (reference is made to the Classical Latin form esse for convenience; however, the Romance paradigms must be taken to derive from a Vulgar Latin form *essĕre, into which other verbs, notably sedēre ‘to sit’ in the case of the Ibero-Romance languages, were suppletively incorporated). The contrastive study of the development of cognate copular verbs in closely related languages needs closer attention in regard to the identification of the parameters of copula choice with adjectival complements.