AJAX is a data cleaning tool that facilitates the specification and execution of data cleaning programs applied to a single source or for integrating multiple data sources. AJAX provides a framework wherein the logic of a data cleaning program is modeled as a directed graph of data transformations which starts from some input source data. AJAX distinguishes four types of data transformations: mapping, matching, clustering and merging transformations. AJAX also provides an expressive and declarative language for specifying data cleaning programs, which consists of SQL statements enriched with a set of specific primitives to express those types of transformations. Each of these primitives corresponds to a transformation whose physical implementation takes advantage of existing RDBMS technology. The declarative nature of the language provides opportunities for automatic optimization and facilitates the maintenance of a data cleaning program. AJAX is extensible in various ways. A rich interactive environment is supplied to the user in order to resolve errors and inconsistencies that cannot be automatically handled and it supports a stepwise refinement design of data cleaning programs.