Assigning numbers to variations in the behaviors, experiences, or beliefs of individuals and groups is a firmly established tradition in Western culture. In many areas of occupational therapy practice, quantitative measures are required to document need for services and, increasingly, quantitative measures are required to document the value of these services in terms of the outcomes achieved. In order to meet these expectations, occupational therapy frequently has adopted both methods and instruments from other disciplines including psychology, medicine, and education. However, the assumptions and modes of thinking about people that accompany these methods are not always compatible with the values and practice of occupational therapy. This paper explores tensions inherent in the assessment process in a profession that is holistic and humanistic in its orientation. I propose that in order for assessment to serve our goal of supporting health and participation through engagement in occupation we must accept the uncertainty and be vigilant about the biases in thinking that are inherent in our measures.