The Kagyu school, also transliterated as Kagyü, or Kagyud (Tibetan: བཀའ་བརྒྱུད།, Wylie: bka' brgyud), which translates to "Oral Lineage" or "Whispered Transmission" school, is one of the main schools (chos lugs) of Tibetan (or Himalayan) Buddhism. The Kagyu lineages trace themselves back to the 11th century Indian Mahasiddhas Naropa, Maitripa and the yogini Niguma, via their student Marpa Lotsawa (1012–1097), who brought their teachings to Tibet. Marpa's student Milarepa was also an influential poet and teacher.

The Tibetan Kagyu tradition gave rise to a large number of independent sub-schools and lineages. The principal Kagyu lineages existing today as independent schools are those which stem from Milarepa's disciple, Gampopa (1079–1153), a monk who merged the Kagyu lineage with the Kadam tradition. The Kagyu schools which survive as independent institutions are mainly the Karma Kagyu, Drikung Kagyu, Drukpa Lineage and the Taklung Kagyu. The Karma Kagyu school is the largest of the sub-schools, and is headed by the Karmapa. Other lineages of Kagyu teachings, such as the Shangpa Kagyu, are preserved in other schools.

The main teachings of the Kagyus include Mahamudra and the Six Dharmas of Naropa.

Strictly speaking, the term bka' brgyud "oral lineage", "precept transmission" applies to any line of transmission of an esoteric teaching from teacher to disciple. There are references to the "Atiśa kagyu" for the Kadam or to "Jonang kagyu" for the Jonang and "Ganden kagyu" for the Gelug sects. Today, however, the term Kagyu almost always refers to the Dagpo Kagyu and, less often, to the Shangpa Kagyu.

In his 1970 article Golden Rosaries of the Bka' brgyud schools, E. Gene Smith discusses the two forms of the name, Wylie: bka' brgyud and Wylie: dkar brgyud:

A note is in order regarding the two forms Dkar brgyud pa and Bka' brgyud pa. The term Bka' brgyud pa simply applies to any line of transmission of an esoteric teaching from teacher to disciple. We can properly speak of a Jo nang Bka' brgyud pa or Dge ldan Bka' brgyud pa for the Jo nang pa and Dge lugs pa sects. The adherents of the sects that practice the teachings centering around the Phyag rgya chen po and the Nā ro chos drug are properly referred to as the Dwags po Bka' brgyud pa because these teachings were all transmitted through Sgam po pa. Similar teachings and practices centering around the Ni gu chos drug are distinctive of the Shangs pa Bka' brgyud pa. These two traditions with their offshoots are often incorrectly referred to simply as Bka' brgyud pa. Some of the more careful Tibetan scholars suggested that the term Dkar brgyud pa be used to refer to the Dwags po Bka' brgyud pa, Shangs pa Bka' brgyud pa and a few minor traditions transmitted by Nā ro pa, Mar pa, Mi la ras pa, or Ras chung pa but did not pass through Sgam po pa. The term Dkar brgyud pa refers to the use of the white cotton meditation garment by all these lineages. This complex is what is normally known, inaccuratly, as the Bka' brgyud pa. Thu'u kwan Blo bzang chos kyi nyi ma sums up the matter: "In some later 'Brug pa texts the written form 'Dkar brgyud' indeed appears, because Mar pa, Mi la, Gling ras, and others wore only white cotton cloth. Nevertheless, it is fine if [they] are all called Bka' brgyud." At Thu'u kwan's suggestion, then, we will side with convention and use the term "Bka' brgyud."

One source indicates:

[T]he term "Kagyu" derives from the Tibetan phrase meaning "Lineage of the Four Commissioners" (ka-bab-shi'i-gyu-pa). This four-fold lineage is