The World Wide Web Consortium (W3C) has issued XML Schema as a W3C Candidate Recommendation. Advancement of the document to Candidate Recommendation is an invitation to the Web development community at large to make implementations of XML Schema and provide technical feedback. Simply defined, XML Schemas define shared markup vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents. The XML Schema specification consists of three parts. One part defines a set of simple datatypes, which can be associated with XML element types and attributes; this allows XML software to do a better job of managing dates, numbers, and other special forms of information. The second part of the specification proposes methods for describing the structure and constraining the contents of XML documents, and defines the rules governing schema-validation of documents. The third part is a primer, which explains what schemas are, how they differ from DTDs, and how someone builds a schema. By bringing datatypes to XML, XML Schema increases XML’s power and utility to the developers of electronic commerce systems, database authors and anyone interested in using and manipulating large volumes of data on the Web. By providing better integration with XML Namespaces, it makes it easier than it has ever been to define the elements and attributes in a namespace, and to validate documents which use multiple namespaces defined by different schemas. XML Schema introduces new levels of flexibility that may accelerate the adoption of XML for significant industrial use. For example, a schema author can build a schema that borrows from a previous schema, but overrides it where new unique features are needed. XML Schema also provides a way for users of e-commerce systems to choose which XML Schema they use to validate elements in a given namespace, thus providing better assurance in e-commerce transactions and greater security against unauthorized changes to validation rules. The working group members include: Academia Sinica; ArborText, Inc; Bootstrap Alliance and LSU; Calico Commerce; Commerce One; Defense Information Systems Agency (DISA); DevelopMentor; Distributed Systems Technology Centre (DSTC Pty Ltd); Graphic Communications Association; Health Level Seven; Hewlett Packard Company; IBM; Informix; Intel; Lawrence Berkeley National Laboratory; Lexica LLC; Lotus Development Corporation; Microsoft Corporation; Microstar; MITRE; NCR; Oracle Corp.; Progress Software; SAP AG; Software AG; Sun Microsystems; TIBCO Software; University of Edinburgh; webMethods, Inc; Xerox; and XMLSolutions. www.w3.org