Rabbi 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 HaRav Abraham 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.

Rav Kook was born in Griva, Russia, in 1865. 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, Rav Kook married Batsheva, the daughter of Rabbi Eliyahu David Rabinowitz-Teomim, (also known as the Aderet), the rabbi of Ponevezh and later Chief Ashkenazi Rabbi of Jerusalem. In 1887, at the age of 23, Rav 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 Rav Kook became the rabbi of Bausk. 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, Rav 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 Rav 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 (Mahzikei Hadath, "upholders of the law"), an immigrant Orthodox community located in Brick Lane, Whitechapel. Upon returning to Eretz Israel, he was appointed the Ashkenazi Rabbi of Jerusalem, and soon after, as first Ashkenazi Chief Rabbi of Palestine in 1921. Rav 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.

Rabbi Abraham Itzhak HaCohen Kook call to the Jewish People on Aliyah: "Come to Eretz Israel, dear brethren, and save your own souls, as well as those of your generations, and of our entire nation. Deliver our Homeland from waste and desolation, degradation and rot. Save it from every kind of defilement and corruption, sorrow and distress, which threaten them in all their far flung communities, without any exception. So do come to Eretz Israel, beloved brethren, and do all in your power to blazen a trek for the return of those of our people who are hated and baited, tainted and hunted. Lead those who return, showing by your example that the road has now been concluded. It is idle to seek other paths, for there is only one way on which we must tread, that is to Eretz Israel."

Rav Kook tried to build and maintain channels of communication and political alliances between the various Jewish sectors, including the secular Jewish Zionist leadership, the Religious Zionists, and other 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 agents 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 process 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 non-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. Sonnenfeld, who, despite disagreeing with Rav Kook on philosophical issues, had a deep respect for him. Rav 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 practicing rejection , he preferred to fill the role of one who embraces. However, Rav Kook was critical of the secularists on certain occasions when they went "too far" in desecrating the Torah, for instance, by not observing the Shabbat or kosher laws. Rav Kook also opposed the secular spirit of the Hatikvah anthem, and penned another anthem with a more religious theme entitled haEmunah ("the faith").

	Eternally lives in our hearts, the loyal faith [haEmunah] to return to our holy land, the city where David settled.
	There we shall work our inheritance, [which the] father of many [nations] acquired, there we shall live our life the life of the innumerable community.
	There we shall serve our G-d with joy, happiness and song there we shall pilgrimage three times a year.
	Torah of life is our desire, given from heavenly mouth forever it is our heritage from the desert it was given.

Roshei Yeshiva following Rav Kook's passing in 1935 included Rav Yaakov Moshe Charlap, Rav Shlomo Raanan, and Rav Kook's son, Rav Tzvi Yehuda Kook.

While Rabbi Kook is exalted as one of the most important thinkers in mainstream Religious Zionism, he was close to what is now called Hardal. Indeed, there are several prominent quotes in which Rav Kook is quite critical of the more modern orthodox Religious Zionists (Mizrachi), whom he saw as naive and perhaps hypocritical in attempting to synthesize traditional Judaism with a modern and largely secular ideology. Rav Kook never shied away from criticizing his peers, religious and secular, as well as the increasingly cloistered traditionalists living in the Holy Land, whose way of life he characterized as being similarly affected by the negative and abnormal conditions of the Jewish exile, and therefore just as "inauthentic" as that of their Zionist counterparts.

Rav Kook was interested in outreach and cooperation between different groups and types of Jews, and saw both the good and bad in each of them. His sympathy for them as fellow Jews and desire for Jewish unity should not be misinterpreted as any inherent endorsement of all their ideas. That said, Rav Kook's willingness to engage in joint projects (for instance, his participation in the Chief Rabbinate) with the secular Zionist leadership must be seen as differentiating him from many of his traditionalist peers. In terms of practical results, it would not be incorrect to characterize Rav Kook as being a Zionist, believing in the re-establishment of the Jewish people as a nation in their ancestral homeland. Unlike other Zionist leaders, however, Rav Kook's motivations were purely based on Jewish Law and Biblical prophecy. His sympathy towards the Zionist movement can be seen as a major stepping stone to the Religious Zionist movement gaining momentum and legitimacy after his death.

The Israeli moshav Kfar Haroeh, founded in 1933, was named after Rav Kook, "Haroeh" being a Hebrew acronym for "HaRav Abraham HaCohen". His son Zvi Yehuda Kook, who was also his most prominent student, took over teaching duties at Mercaz HaRav after his death, and dedicated his life to disseminating his father's philosophy. Rav Kook's writings and philosophy eventually gave birth to the Hardal Religious Zionist movement which is today led by rabbis who studied under Rav Kook's son at Mercaz HaRav.