Rabbi Israel Isidore Mattuck (1884–1954) was a leader of the Liberal (Reform movement) in the United Kingfom. Born in Lithuania, he came as a child to the United States with his family and grew up in Worcester, Massachusetts. A graduate of Harvard University, he was ordained at the Hebrew Union College in 1910 having only spent two years in residence. He held a pulpit in Far Rockaway, New York, for a year and then went to England to serve a young congregation, the Liberal Synagogue in London. He served as senior minister for 36 years and then after 1947 was minister emeritus. Under his leadership the synagogue grew into one of the largest synagogues in London. The building that he helped build was bombed in World War II, but Mattuck lived to see it restored and rededicated. He was succeeded by his disciple and son-in-law Rabbi Leslie Edgar. He was a leading figure, perhaps the leading figure in English Liberal Jewry, its philosopher and its public face. He was known as one of the "Three Ms": Montagu, Montefiore, and Mattuck. He helped form the Union of Liberal and Progressive Synagogues and helped establish the World Union for Progressive Judaism in 1926 and served as its first chairman from 1926 until his death. He was chairman of the Society of Jews and Christians. He compiled and edited the Liberal prayer book, first in three volumes in 1923–26 and in a revised edition in 1937. He is the author of several books: What Are the Jews (1939); The Essentials of Liberal Judaism (1947); Jewish Ethics (1953); and The Thought of the Prophets (1953). He also edited Aspects of Progressive Jewish Thought (1955), which was dedicated in honor of Leo Baeck's 80th birthday. It was published posthumously.