This book began as a result of a reunion workshop for members of Sound to Sense (S2S), an EU-funded Marie Curie Research Training Network (MC-RTN) that ran from 2007 to 2011. Engineers, computer scientists, psychologists, and linguistic phoneticians worked together to explore new ideas about how humans understand speech, and how scientists can use this knowledge to improve the way we communicate with each other and with machines. We used a variety of approaches to investigate what types of information are available in the speech signal, and how listeners use that information when they are listening in their native language, or in a foreign language, or in a noisy place like a railway station when it is hard to hear the speech. These three types of listening situation allow us to see how listeners actively use their pre-existing knowledge, together with the speech they hear, to understand a message.