We present the Leiden Manifest. We offer this distillation of best practice in metrics-based research assessment so that researchers can hold evaluators to account, and evaluators can hold their indicators to account. Its ten principles are not news to scientometricians, although none of us would be able to recite them in their entirety because codification has been lacking until now. But they are not in the room when evaluators report back to university administrators who are not expert in the relevant methodology. Scientists searching for literature with which to contest an evaluation find the material scattered in what are, to them, obscure journals to which they lack access.