The debate between internalism and externalism has become a focal point of attention both in epistemology and in the philosophy of mind and language. Externalism challenges basic traditional internalist conceptions of the nature of knowledge, justification, thought and language. What is at stake, is the very form that theories in epistemology and the philosophy of mind ought to take. This volume is a collection of original contributions of leading international authors reflecting on the present state of the art concerning the exciting controversies between internalism and externalism.