What Is Property?: or, An Inquiry into the Principle of Right and of Government is a work of nonfiction on the concept of property and its relation to anarchist philosophy by the French anarchist and mutualist Pierre-Joseph Proudhon, first published in 1840. In the book, Proudhon most famously declared that "property is theft".

Pierre-Joseph Proudhon (15 January 1809 – 19 January 1865) was a French politician and the founder of mutualist philosophy. He was the first person to declare himself an anarchist and is widely regarded as one of the ideology's most influential theorists. Proudhon is even considered by many to be the "father of anarchism".He became a member of the French Parliament after the revolution of 1848, whereafter he referred to himself as a federalist.