Jacob Riis was a Danish-American social historian, journalist, and photographer who is best known for his work in documenting the squalid living conditions of New York City's poor in the late 19th century. He is also remembered for his book How the Other Half Lives, which exposed the harsh realities of life in the slums of New York City. Riis was a major influence on the progressive movement of the early 20th century and his work helped to bring about reforms in housing and public health.