In a Health Information Technology (HIT) regulatory context in which the usability of this technology is more and more a critical issue, there is an increasing need for evidence based usability practice. However, a clear definition of evidence based usability practice and how to achieve it is still lacking. This paper underlines the need for evidence based HIT design and provides a definition of evidence based usability practice as the conscientious, explicit and judicious use of current best evidence in making decisions in design of interactive systems in health by applying usability engineering and usability design principles that have proven their value in practice. Current issues that hamper evidence based usability practice are highlighted and steps needed to achieve evidence are presented.