Although document type definitions can enforce basic structural rules on documents, many applications need a more powerful and expressive validation method. The W3C developed the XML Schema Recommendation to address these needs. Schemas can describe complex restrictions on elements and attributes. Multiple schemas can be combined to validate documents that use multiple XML vocabularies. This chapter provides a rapid introduction to key W3C XML Schema concepts and usage, starting with the fundamental structures that are common to all schemas. We begin with a very simple schema and proceed to add more functionality to it until every major feature of XML Schemas has been introduced.