In Romance–Slavic language contact, both language families have had foreign influence, with Romance varieties as donor and as recipient languages. Slavic has been in contact with languages of the Latin phylum at least since the first encounters of South-Slavic tribes with the Balkan–Romance population in the 6th century ce. Mutual language contact became especially visible in South-Slavic influence on Romanian and its South Danubian varieties (Aromanian, Megleno-Romanian, and Istro-Romanian) and also the other way round, in the form of Romance borrowings in the Serbo-Croatian (Bosnian–Croatian–Montenegrin, Serbian) continuum, Bulgarian / Macedonian, and Slovene. However, pre-Balkan contacts of Proto-Slavic with Italic or Latin have also been claimed.
Balkan Latin derived from common Latin and split into Western and Eastern Balkan Romance, forming the basis of local Romance vernaculars, with (extinct) Dalmatian in the west of the peninsula and Proto-Romanian in the east. Proto-Romanian and Old Bulgarian mutually influenced each other, which led to a divergent position of Romanian and Bulgarian / Macedonian in their respective language families. Mutual Romance–Slavic language contact continued even after the Middle Ages, between Romanian, Italo-Romance, French, and Serbo-Croatian, Slovene, Bulgarian / Macedonian.
The vocabulary of all Balkan languages and varieties in contact has been heavily affected by words and concepts of the respective contact languages—in the case of Romanian-based varieties as a donor language by distributing shepherd and dairy terminology throughout South Slavic. As for grammar, Macedonian developed a possessive perfect by copying the Aromanian model.
In the situation of South-Slavic minority languages in all-embracing contact with Italo-Romance in southern and northern Italy, many contact-induced developments occurred, not only in the lexicon but also in the grammatical system. Examples of the effect of 500 years of bilingualism of the Molise Slavs, following immigration from Dalmatia to southern Italy in the 16th century, include the loss of the locative due to the homonymic expression of motion and state in the Italo-Romance donor varieties, the loss of the neuter gender of nouns, and the preservation of a fully functional imperfect. Others are the formation of a new de-obligative future and a venitive passive. Loans were fully integrated in the existing morphological systems, for example, by developing special integration rules for verbs, including a procedure of forming aspectual pairs from telic source verbs. One thousand years of Romance–Slavic contact have had similar effects on Slovene-based Resian in northeastern Italy, although to a lesser extent.
The opposite case of Slavic (Croatian) influence on a Romance microlanguage is found in far-reaching contact-induced changes in Istro-Romanian grammar, such as the rise of a neuter gender and, especially, the development, at least in part, of a Slavic-type aspect category, formally marked by affixes.
The numeral systems of the recipient languages have often been restructured by the influence of their donor languages, resulting, as a rule, in mixed systems with higher numbers (starting from 5) being predominantly of foreign provenience. The Slavic way of counting teens (one on ten, etc.) has spread throughout the Balkans.