Clouds are everywhere today. Massive online retail sites like Amazon and Ebay rely on clouds for reliability and performance. Email and instant messaging are built on clouds. Services like Uber and Spotify would be hard to imagine without clouds. The great search engines like Google and Bing that comb the Internet are implemented on clouds. Social media like Facebook and Reddit are also cloud implementations. The term is everywhere, but exactly what is a cloud? And do clouds compromise our security and privacy? The answers to these questions are complex, but this chapter offers them.