Abraham Isaac Kook (1865–1935) was the first Ashkenazi chief rabbi of the British Mandate for Palestine, the founder of the Religious Zionist Yeshiva Merkaz HaRav, Jewish thinker, Halachist, Kabbalist and a renowned Torah scholar. He is known in Hebrew as הרב אברהם יצחק הכהן קוק HaRav Avraham Yitzchak HaCohen Kook, and by the acronym HaRaAYaH or simply as "HaRav." He was one of the most celebrated and influential Rabbis of the 20th century.

Kook was born in Grīva, Latvia (now part of Daugavpils, then town in Courland, governorate of Imperial Russia) in 1865, the oldest of eight children. His father, Rabbi Shlomo Zalman Ha-Cohen Kook, was a student of the Volozhin Yeshiva, the "mother of the Lithuanian yeshivas", whereas his maternal grandfather was a member of the Kapust dynasty of the Hassidic movement.

As a child he gained a reputation of being an ilui (prodigy). He entered the Volozhin yeshiva in 1884 at the age of 18, where he became close to the rosh yeshiva, Rabbi Naftali Zvi Yehuda Berlin (the Netziv). Although he stayed at the yeshiva for only a year and a half, the Netziv has been quoted as saying that if the Volozhin Yeshiva had been founded just to educate Rav Kook, it would have been worthwhile. During his time in the yeshiva, he studied about 18 hours a day.

In 1886, Kook married Batsheva, the daughter of Rabbi Eliyahu David Rabinowitz-Teomim, (also known as the Aderet), the rabbi of Ponevezh (today's Panevėžys, Lithuania) and later Chief Ashkenazi Rabbi of Jerusalem. In 1887, at the age of 23, Kook entered his first rabbinical position as rabbi of Zaumel, Lithuania. In 1888, his wife died, and his father-in-law convinced him to marry her cousin, Raize-Rivka, the daughter of the Aderet's twin brother. In 1895 Kook became the rabbi of Bausk (now Bauska). Between 1901 and 1904, he published three articles which anticipate the fully-developed philosophy which he developed in the Land of Israel. During these years he wrote a number of works, most published posthumously, most notably a lengthy commentary on the Aggadot of Tractates Berakhot and Shabbat, titled 'Eyn Ayah' and a brief but powerful book on morality and spirituality, titled 'Mussar Avikhah'.

In 1904, Kook moved to Ottoman Palestine to assume the rabbinical post in Jaffa, which also included responsibility for the new mostly secular Zionist agricultural settlements nearby. His influence on people in different walks of life was already noticeable, as he engaged in kiruv ("Jewish outreach"), thereby creating a greater role for Torah and Halakha in the life of the city and the nearby settlements.

The outbreak of the First World War caught Kook in Europe, and he was forced to remain in London and Switzerland for the remainder of the war. In 1916, he became rabbi of the Spitalfields Great Synagogue (Machzike Hadath, "upholders of the law"), an immigrant Orthodox community located in Brick Lane, Whitechapel. While there, he was involved in the activities which led to the Balfour Declaration, 1917. Upon returning, he was appointed the Ashkenazi Rabbi of Jerusalem, and soon after, as first Ashkenazi Chief Rabbi of Palestine in 1921. Kook founded a yeshiva, Mercaz HaRav Kook (popularly known as "Mercaz haRav"), in Jerusalem in 1924. He was a master of Halakha in the strictest sense, while at the same time possessing an unusual openness to new ideas. This drew many religious and non­religious people to him, but also led to widespread misunderstanding of his ideas. He wrote prolifically on both Halakha and Jewish thought, and his books and personality continued to influence many even after his death in Jerusalem in 1935.

Kook built bridges of communication and political alliances between the various Jewish sectors, including the secular Jewish Zionist leadership, the Religious Zionists, and more traditional non-Zionist Orthodox Jews. He believed that the modern movement to re-establish a Jewish state in the land of Israel had profound theological significance and that the Zionists were pawns in a heavenly plan to bring about the messianic era. Per this ideology, the youthful, secular and even anti-religious Labor Zionist pioneers halutzim were a part of a grand divine scheme whereby the land and people of Israel were finally being redeemed from the 2,000 year exile (galut) by all manner of Jews who sacrificed themselves for the cause of building up the physical land, as laying the groundwork for the ultimate spiritual messianic redemption of world Jewry. He once commented that the establishment of the Chief Rabbinate was the first step towards the re-establishment of the Sanhedrin.

His empathy towards the anti-religious elements aroused the suspicions of his more traditionalist haredi opponents, particularly that of the traditional rabbinical establishment that had functioned from the time of Turkey's control of greater Palestine, whose paramount leader was Rabbi Yosef Chaim Sonnenfeld, Kook's greatest rabbinical rival. Kook once quoted a rabbinic axiom that "one should embrace with the right hand and rebuff with the left". He remarked that he was fully capable of rejecting, but since there were enough rejecters, he was fulfilling the role of embracer. However, Kook was critical of the secularists on certain occasions when they went "too far" in desecrating the Torah, for instance, by not observing the Sabbath or kosher laws. Kook also opposed the secular spirit of the Hatikvah anthem, and penned another anthem with a more religious theme entitled haEmunah.

Kook fathered three children through his two wives: two daughters and a son, Rabbi Zvi Yehuda Kook. His nephew was Hillel Kook.