In the popular perception of the Rockefeller dynasty, John D. Rockefeller, Sr. and his grandson Nelson have always occupied center stage. And yet it was the man in the middle, John D. Rockefeller, Jr. (also known as "Junior") who radically changed the very identity of the family and the impact of its legacy. He made "Rockefeller" synonymous with philanthropy and social causes at a time when public opinion was anything but sympathetic to that name. Self-critical and subdued, but also rigid and determined, he embodied many of the contradictions that would characterize the Rockefellers' private and public life.

Junior was born in Cleveland, Ohio, on January 29, 1874 — the youngest of four, and the only son born to the founder of Standard Oil. Along with sisters Alta, Bessie, and Edith, he was educated at home until the age of 10. Made to wear his sisters' hand-me-downs for the first few years of his life, Junior quickly absorbed the strict discipline and frugal lifestyle inculcated by his parents. "My mother and father raised but one question: Is it right, is it duty?" he would recall years later. "I took responsibility early, and, like my parents, I was serious."

In 1884, the Rockefellers moved to New York and Junior began regular schooling. He was a bright and earnest student, although quite insecure. At 13, he developed stress-related disorders, suffering the first of a series of breakdowns that would trouble him for the rest of his life. During his adolescence, he grew extremely attached to his father and mother, who reciprocated with a mixture of intense affection and steadfast moral indoctrination.

In 1893, the young heir found himself in a quite a different environment, one that fostered a new openness to the outside world and brought him in touch with people and ideas that would have an enormous impact in years to come. At Brown University in Providence, Rhode Island, he learned about the new theories that applied scientific knowledge to the solution of economic and social problems and was influenced by the spirit of reform that prevailed on campus. He also met Abby Aldrich, the daughter of a powerful U.S. senator from Rhode Island, and a charismatic woman who loved art and was not impressed by his money.

After four years of vacillations and self-doubt, Junior finally proposed. The couple was wed at a lavish ceremony in Senator Aldrich's estate in Warwick, Rhode Island, on October 9, 1901. One thousand guests attended, as newspapers across the country touted the new alliance between wealth and political power. More than that, Abby brought a new vitality to the Rockefellers and a different perspective on life within and without the confines of the family's estate.

This difference in temperament was particularly apparent in the couple's relationship to their offspring: Abby (known as "Babs", 1903), John D. III (1906), Nelson (1908), Laurance (1910), Winthrop (1912), and David (1915). Concerned about the corrupting influence of wealth on his family, Junior was a very severe father. He insisted that the children keep ledgers to account for their allowances and that they only spend a third of the money earned by performing numerous chores (the other two thirds were to be donated and saved). With Abby, he had an intense and often possessive relationship, relying on her frequently for advice.

In 1896, Junior entered the Standard Oil offices at 26 Broadway. "I feel but little confidence in my ability to fill the position which is before me," he wrote in a letter to his mother, "but know that I am not afraid to work or do whatever is required of me, and with God's help I will do my best." Junior endeavored to find a role for himself in his father's empire, but soon grew disenchanted with the business and the controversies that had erupted around it in the early 1900s. In 1910, at the age of 36, he decided to cut all ties with the Standard Oil trust (except for two of its companies) and devote himself to philanthropy.

It wasn't long before his determination to change the family's image suffered its most serious setback. In September 1913, 9,000 workers at the Rockefeller-owned Colorado Fuel and Iron Company went out on strike for union recognition, better hours, wages, and housing. The coal miners and their families, evicted from their company homes, spent the winter in tent colonies in the town of Ludlow. In March 1914, with the strike still unresolved, John D. Rockefeller, Jr., a board member of the company, testified before the House Subcommittee on Mines and Mining. He upheld the "open shop" as a sacred ideal. When the strike was violently suppressed in April 1914, public opinion turned against Junior, blaming him for the deaths of two women and 10 children. With the help of labor expert William MacKenzie King, Junior confronted the crisis and gradually addressed some of the issues raised by the miners. His efforts culminated in a much-publicized visit to Ludlow and a new labor agreement that improved conditions at the Colorado mine.

Moved by a sense of stewardship, his deeply held religious beliefs, and his interest in scientific and social progress, Junior initiated and pushed forward a large number of philanthropic endeavors. In 1901, he collaborated with his father in the creation of the Rockefeller Institute for Medical Research (now Rockefeller University), one of the first medical research institutions in the nation. A year later, he established the General Board of Education, dedicated to the improvement of the educational system in the South. He was responsible for the restoration of Colonial Williamsburg, started in 1926, and changed the layout of New York City with the construction of the Rockefeller Center (1931-1940). Further additions to New York's cityscape were Riverside Church, symbolic of Junior's attempt to create an interfaith Protestant movement, and The Cloisters, fashioned from elements from five medieval monasteries brought over from Europe to house the Metropolitan Museum of Art's medieval collection.

Junior also showed a particular interest in the area of conservation, donating tens of thousands of acres to the Grand Teton National Park in Wyoming and the Acadia National Park in Maine. In 1946, he gave eight million dollars for the purchase of the land for the permanent home of the United Nations in New York. His charitable gifts are estimated to have totaled $537 million, with the largest portion going to general foundations; churches and religious organizations; historic preservation and restoration; colleges and universities; parks and conservation; and libraries and museums, in that order.

After the death of Abby in 1948, Junior married Martha Baird Allen, the widow of a former college friend. The postwar years saw him taking on a less prominent role, as he sold the Rockefeller Center to his five sons in 1948 and handed down some of his fortune through numerous financial gifts to them. He died in Tucson, Arizona, in 1960, at the age of 86.