"What to measure" refers to domains stable over time. "How to measure" is constantly evolving. Lacking a common terminology and common underlying conceptual model of functioning and disability, what and how to measure have been described differently in the various OMERACT Core Sets. With the approval of the International Classification of Functioning, Disability and Health (ICF) by the World Health Assembly in 2001, we now have a universally conceptual model that integrates the biomedical and societal model of functioning and disability. The so-called ICF Core Sets can be used as a basis for the further specification of OMERACT domains addressing aspects of functioning. In line with the successful approach taken by OMERACT, it is suggested to comprehensively specify the domain "function" when defining "what should be measured," and only then to recommend how to measure or which health status measure to use. We recommend comparing the specifications of domains addressing aspects of functioning of OMERACT Core Sets already established with the ICF Core Sets, and examine whether the ICF Core Sets may be useful for the further specification of these domains.