A flow meter generates flow data - which contains information about each connection observed on a network - from a stream of observed packets. Flow meters can be implemented in standalone measurement devices or inline on packet forwarding devices, such as routers. YAF (Yet Another Flowmeter) was created as a reference implementation of an IPFIX Metering and Exporting Process, and to provide a platform for experimentation and rapid deployment of new flow meter capabilities. Significant engineering effort has also gone into ensuring that YAF is a high performance, flexible, stable, and capable flow collector. This paper describes the some of the issues we encountered in designing and implementing YAF, along with some background on some of the technologies that we chose for implementation. In addition we will describe some of our experiences in deploying and operating YAF in large-scale networks.