The focus of this contribution is on the theoretical principles and concepts behind evaluation of IT-based systems, discussing their presuppositions, implications and interrelationships; for instance in relation to a series of issues to consider: terminology for the concepts used as that is a reason for many disputes, bias as that is a common reason for less accuracy and trustworthiness in conclusions, culture as the tacit driver of everything we do and design, constructive evaluation as this has strict time and timing issues, preparing for meta-analyses as that is in the near future, and top-level issues in choice of methodology. Awareness in these respects will lead to avoidance of major pitfalls and perils at evaluation and thereby improve the validity and trustworthiness of an evaluation outcome, supporting the initiative towards evidence-based health informatics.