Roger Dummer Fisher, a Harvard Law School professor who devoted his professional life to conflict resolution but spent half a century of summers enjoying the peace of Watcha Pond in rural West Tisbury, died in Hanover, N.H., on August 25 at the age of 90.

Co-author of the best-selling book, Getting to Yes: Negotiating Conflict Without Giving In, Mr. Fisher at the time of his death was Williston professor of law emeritus at the law school, where he co-founded the Harvard Negotiation Project. Married for 62 years to the former Caroline McMurtrie Speer, he is survived by two sons, Elliott and Peter. His wife died two years ago.

A leading innovator in the theory, teaching and practice of negotiation and conflict resolution, Mr. Fisher’s contribution to the field was based on several key insights: that peace is not a piece of paper, but a way of dealing with conflict when it arises; that the process used to negotiate affects the likely outcome; and that seeking creative ways to increase the size of the pie instead of simply splitting it produces the best results.

On his 80th birthday, Mr. Fisher’s friend and colleague at Harvard, the late John Kenneth Galbraith said of him: “Whenever I thought, ‘Someone should do something about this,’ it eased my conscience to learn that Roger was already working on it.”

Born in Winnetka, Ill., on May 28, 1922 to Walter T. and Katherine Fisher, Mr. Fisher volunteered to serve in the U.S Air Force in World War II, an experience that informed his life’s work. A graduate of Harvard and Harvard Law School, he worked on Ambassador Averell Harriman’s Marshall Plan staff in Paris and practiced law in Washington before joining the faculty at Harvard Law School in 1958.

His son Peter said his half-century love affair with Martha’s Vineyard began that year when the family stayed on Chilmark Pond in a house owned by another Harvard Law School professor, Louis Jaffe.

“He literally walked up and down South Beach to find a piece of property,” Peter said.

In 1960, he purchased the former site of the historic Watcha Gun Club at Scrubby Neck, a 60-acre parcel situated between Watcha Pond and the ocean in West Tisbury, from Richard I. Keeler. For several years the family lived in the old clubhouse before subdividing the property and building their own home, his son said. Later, Mr. Fisher built a one-room study in the woods where he would retreat to do his writing.

“He had a deep affection for the Island and especially Watcha Pond,” his son told the Gazette. “I think it connected to his boyhood going canoeing in the Canadian wilderness, and here it was right on the beach.”

Though he wrote more than a dozen books, it was Getting to Yes that introduced Mr. Fisher’s ideas to a generation of political and business leaders. The book has sold eight million copies over three editions and has been translated into 36 languages. Reviewing its impact after 10 years, the National Institute of Dispute Resolution commented: “No other book in the field comes close to its impact on the way practitioners, teachers, researchers, and the public approach negotiation.”

Over a long and distinguished career, Mr. Fisher tirelessly intervened in a wide range of international disputes, often offering advice and facilitating conversations that led to major breakthroughs. He is credited with assisting in the Camp David peace accords between Egypt and Israel, in pushing for better U.S-Soviet relations, in helping to end civil war in El Salvador, to laying groundwork for the end of apartheid in South Africa and for brokering peace between Ecuador and Peru.

In a tribute published by Harvard, Bruce Patton, who co-wrote the 1991 edition of Getting to Yes, said, “Roger sought unabashedly to change the world, and he did so profoundly. More important, his ideas will continue to spread on their own and through the efforts of his many devoted students in the field he helped create.”