Peer-to-peer (P2P) computing offers new opportunities for building highly distributed data systems. Unlike client-server computing, P2P is a very dynamic environment where peers can join and leave the network at any time and offers important advantages such as operation without central coordination, peers autonomy, and scale up to large number of peers. However, providing high-level data management services (schema, queries, replication, availability, etc.) in a P2P system implies revisiting distributed database technology in major ways. In this chapter, we discuss the design and implementation of high-level data management services in APPA (Atlas Peer-to-Peer Architecture). APPA has a network-independent architecture that can be implemented over various structured and super-peer P2P networks. It uses novel solutions for persistent data management with updates, data replication with semantic-based reconciliation and query processing. APPA's services are implemented using the JXTA framework.