It is difficult to imagine a world without science policy. Ever since Vannevar Bush published his seminal insights on the role of science in society, science policy has become deeply ingrained in public policy. Alongside this, the discipline of scientometrics developed. It started from library and information needs, helping the ever-growing scientific community to access, retrieve and disseminate its ever-increasing output. However, along the way, scientometrics developed into a powerful set of scientifically validated data, indicators and tools. It diffused across many disciplines in the social sciences. Over time, this evolution came to the attention of policymakers. The wealth of data and indicators developed in the field of scientometrics (later extended to informetrics and webometrics) elicited interest in their use for policy purposes. A symbiosis between scientometrics and science policy was born. Using the case of the Flemish Centre for Research & Development Monitoring (ECOOM), we describe and illustrate this coevolution between scientometrics and science policy, its opportunities and its challenges, and its do's and don'ts.