Publisher Summary This chapter gives an introduction to the type system of XML schema. It begins with a very simple example, and then moves into the type system of XML schema, discussing simple and complex types. XML schema data types have also been adopted by other schema languages such as Relax NG. Simple data types in XML schema are constructed by means of a few basic concepts: value space, lexical space, fundamental facets, constraining facets, and type extensions. This chapter discusses these basic concepts and then presents the data types that are built into XML schema. It closes with a discussion on the structure in XML schema. XML documents have a hierarchical structure. There is a root element that has child elements that in turn can have other child elements, and so on. The root element of a document is defined on the schema level via an element definition. On the schema level, two types of elements can be defined: the first element definition specifies the root element for all document instances, and subsequent element declarations on the schema level are used to specify global elements.