This document describes policy-based accounting which is an approach to provide flexibility to accounting architectures. Accounting policies describe the configuration of an accounting architecture in a standardized way. They are used to instrument the accounting architecture and can be exchanged between Authentication, Authorization and Accounting (AAA) entities in order to share configuration information. This document describes building blocks and message sequences for policy-based accounting in the generic AAA architecture (RFC 2903). Examples are given for the usage of accounting policies in different scenarios. It is also shown how accounting components can be integrated into the AAA authorization framework (RFC 2904). This document does not propose a language for the description of accounting policies. Rather, it is assumed that a suitable policy language can be chosen from existing or upcoming standards.