Jacob Riis was a journalist and reformer around the turn of the century in New York. He published a book of his photographs and descriptions of the harsh conditions of life in the Lower East Side tenements, How the Other Half Lives, in 1890.
Many of these cramped indoor spaces and dark alleys had never been captured on film, and allowed many people to see the homes of the mostly immigrant populations for the first time. Riis advocated for the city to improve the drinking water supply and other reforms to poor neighborhoods. There are several parks and buildings named after him across the city today.
These photos are from How the Other Half Lives, which is now in the public domain (the ebook version $0.99 if you want to check it out).