Enormous amounts of data exist in "well-behaved" formats such as relational tables and XML, which come equipped with extensive tool support. However, vast amounts of data also exist in non-standard or ad hoc data formats, which often lack standard or extensible tools. This deficiency forces data analysts to implement their own tools for parsing, querying, and analyzing their ad hoc data. The resulting tools typically interleave parsing, querying, and analysis, obscuring the semantics of the data format and making it nearly impossible for others to resuse the tools. This proposal describes PADS, an end-to-end system for processing ad hoc data sources. The core of PADS is a declarative language for describing ad hoc data sources and a data-description compiler that produces customizable libraries for parsing the ad hoc data. A suite of tools built around this core includes statistical data-profiling tools, a query engine that permits viewing ad hoc sources as XML and for querying them with XQuery, and an interactive front-end that helps users produce PADS descriptions quickly.