Watcher is an open source software package which provides a flexible and scalable resource optimization service for multi-tenant OpenStack-based clouds. Watcher provides a complete optimization loop. This provides a robust framework to realize a wide range of cloud optimization goals, including the reduction of data center operating costs, increased system performance via intelligent virtual machine migration,increased energy efficiency, etc... Watcher provides several out-of-box optimization routines for immediate value-add, but it also supports a pluggable architecture. The overall goal is that OpenStack-based clouds equipped with Watcher will decrease their Total Cost of Ownership (TCO) by way of more efficient use of their infrastructure through targeted optimizations and close-loop automation.