Kajkavian[a] is a South Slavic supradialect or language spoken primarily by Croats in much of Central Croatia and Gorski Kotar.

[5] There are differing opinions over whether Kajkavian is best considered a dialect of the Serbo-Croatian language or a fully-fledged language of its own, as it is only partially mutually intelligible with either Čakavian or Štokavian and bears more similarities to Slovene; it is transitional to and fully mutually intelligible with Prekmurje Slovene and the dialects in Slovenian Lower Styria's region of Prlekija in terms of phonology and vocabulary.

Autonyms used throughout history by various Kajkavian writers have been manifold, ranging from Slavic (slavonski, slovenski, slovinski) to Croatian (horvatski) or Illyrian (illirski).

Some Slavists maintain that when the separation of Western South Slavic speeches happened, they separated into five divergent groups — Slovene, Kajkavian, Čakavian, Western Štokavian and Eastern Štokavian, as a result of this, throughout history Kajkavian has often been categorized differently, either a node categorized together with Serbo-Croatian or Slovene.

[16][17][18] Croatian linguist Stjepan Ivšić has used Kajkavian vocabulary and accentuation, which significantly differs from that of Štokavian, as evidence to be a language in its own right.

[19] Josip Silić, one of the main initiators behind the standardisation of Croatian, also regards Kajkavian as a distinct language by dint of its having significantly different morphology, syntax and phonology from the official Štokavian-based standard.

[20] However, Silić's theorization about three languages and systems of Croatian, based on Ferdinand de Saussure and Eugenio Coșeriu concepts, is criticized for being exaggerated, incomprehensible and logically non-existent.

[15] The Croatian capital, Zagreb, has historically been a Kajkavian-speaking area, and Kajkavian is still in use by its older and (to a lesser extent) by its younger population.

[26] The vast intermingling of Kajkavian and standard Štokavian in Zagreb and its surroundings has led to problems in defining the underlying structure of those speech-groups.

As modern-day Hungary used to be populated by Slavic-speaking peoples prior to the arrival of Hungarians, there have been hypotheses on possible common innovations of future West and South Slavic speakers of that area.

Due to the great diversity within Kajkavian primarily in phonetics, phonology, and morphology, the Kajkavian dialect atlas features a large number of subdialects: from four identified by Ivšić to six proposed by Croatian linguist Brozović (formerly the accepted division) all the way up to fifteen according to a monograph by Croatian linguist Mijo Lončarić (1995).

The mixed half-Kajkavian towns along the eastern and southern edge of the Kajkavian-speaking area are Pitomača, Čazma, Kutina, Popovača, Sunja, Petrinja, Martinska Ves, Ozalj, Ogulin, Fužine, and Čabar, including newer Štokavian enclaves of Bjelovar, Sisak, Glina, Donja Dubrava and Novi Zagreb.

The southernmost Kajkavian villages are Krapje at Jasenovac; and Pavušek, Dvorišče and Hrvatsko selo in Zrinska Gora (R. Fureš & A. Jembrih: Kajkavski u povijesnom i sadašnjem obzorju p. 548, Zabok 2006).

The major cities in northern Croatia are located in what was historically a Kajkavian-speaking area, mainly Zagreb, Koprivnica, Krapina, Križevci, Varaždin, Čakovec.

The typical archaic Kajkavian is today spoken mainly in Hrvatsko Zagorje hills and Međimurje plain, and in adjacent areas of northwestern Croatia where immigrants and the Štokavian standard had much less influence.

Other southeastern people who immigrate to Zagreb from Štokavian territories often pick up rare elements of Kajkavian in order to assimilate, notably the pronoun "kaj" instead of "što" and the extended use of future anterior (futur drugi), but they never adapt well because of alien eastern accents and ignoring Kajkavian-Čakavian archaisms and syntax.

[60] The first comprehensive works in Kajkavian started to appear during the 16th century at a time when Central Croatia gained prominence due to the geopolitical environment since it was free from Ottoman occupation.

[63] Kajkavian began to lose its status during the Croatian National Revival in mid-19th Century when the leaders of the Illyrian movement opted to use the Štokavian dialect as the basis for the future South Slavic standard language, the reason being that it had the highest number of speakers.

With the notable exception of vocabulary influence of Kajkavian on the standard Croatian register (but not the Serbian one), there was very little to no input from other non-Štokavian dialects.

[65] Instead, the opposite was done, with some modern-day linguists calling the process of 19th-century standardization an event of "neo-Štokavian purism" and a "purge of non-Štokavian elements".

[30] Early 20th century witnessed a drastic increase in released Kajkavian literature, although by then it had become part of what was considered Croatian dialectal poetry with no pretense of serving as a standard written form.

The most notable writers of this period were among others, Antun Gustav Matoš, Miroslav Krleža, Ivan Goran Kovačić, Dragutin Domjanić and Nikola Pavić.

Kruhek naš vsakdaneši daj nam denes ter odpuščaj nam duge naše, kakti i mi odpuščamo dužnikom našim, ter naj nas vpelati v skušnje, nek zbavi nas od vsakih hudobah.

[citation needed] With the recent regional democratizing and cultural revival beginning in the 1990s, Kajkavian partly regained its former half-public position chiefly in Zagorje and Varaždin Counties and local towns, where there is now some public media e.g.: