Jacob Riis was a Danish-American social reformer and journalist who is best known for his work in documenting the squalid living conditions of New York City's poor. He is also credited with helping to spark the Progressive Era in the United States. His most famous book, How the Other Half Lives, was published in 1890 and exposed the harsh realities of life in the slums of New York City. Riis was also an advocate for public health reform and helped to create the first public playgrounds in the city.