In this paper, we are concerned with the distributed monitoring of P2P systems. We introduce the P2P Monitor system and a new declarative language, namely P2PML, for specifying monitoring tasks. A subscription is compiled into a distributed algebraic plan which is described using an algebra over XML streams. We introduce a filter for streams of XML documents that scales by processing first simple conditions and then, if still needed, evaluating complex queries. We also show how particular tasks can be supported by identifying subtasks already provided by existing streams.