In the aftermath of the Second World War, Karl Popper introduced his concept of the 'open society'. Poppers focussed primarily on communism and fascism. Nowadays, communism and fascism have faded away as the primary challenges for open, democratic states, but new challenges have come to the fore. Closed communities pose a significant challenge and sometimes a threat to democratic values. What are those closed communities? This book presents a diagnosis of those closed communities, their problems, and the challenges for an open society.