Jacob Riis was a Danish-American journalist, social reformer, and photographer. He is best known for his work in documenting the plight of the poor in New York City during the late 19th century. His book, How the Other Half Lives, exposed the squalid living conditions of the city's slums and helped to bring about reforms in housing and public health. He also wrote several other books on social reform, including The Children of the Poor and Out of Mulberry Street. Riis was an influential figure in the Progressive Era and his work continues to be relevant today.