The second generation DAL protocols incorporate more comprehensive metadata and data models than previously, as well as functional enhancements for posing more general queries, describing the data thus discovered, and accessing data via robust web and grid technologies. Standard generic dataset metadata has been defined which can be used to uniformly describe any type of astronomical data. The query interface has been generalized to support queries by a variety of dataset attributes, hence queries no longer have to be based upon spatial position on the sky. The coordinate systems and reference frames used have been formalized.

In this chapter we review the currently planned hierarchy of DAL services representing the major classes of astronomical data. Each major class of data, and the associated data model and access protocol, is briefly covered. We present a special section on SSAP, the first of the second generation DAL services, which introduces much of the approach and technology to be used for the second generation services. Finally we introduce the VOClient, DALClient, and DALServer implementations from NVO, which provide reference implementations of the DAL protocols for both the client and server side of things.