What you will get out of this chapter (most often in combination with other chapters referenced here) will depend on how you expect to use ADQL. If you are a general VO user, you will primarily find yourself using ADQL through a portal interface to search catalogs in pursuit of science. For you, the goals of this chapter are to be able to create original ADQL queries using the Open SkyQuery Portal and, in particular, to use ADQL to crossmatch sources from two catalogs. If you are a “scripter”, a user capable of creating custom scripts to do VO research, you may find it useful to use client libraries to query distributed catalogs. If you are a data provider, then you might be interested in deploying a service that uses ADQL (e.g. a SkyNode service). For scripters and data providers alike, our goals are to understand the role of the two formats for ADQL in supporting query web services and to see how the language will eventually be used with other services beyond SkyNodes. If you are a developer, you may wish to incorporate support for ADQL into new applications or services; our goals then are to be able to use the ADQL library to convert between its two formats and to understand how it can be the ADQL library can be adapted to a new local query language.