Chapter 3 addresses ‘what is the problem?’. In other words, the target behaviour or symptom that is of concern and which is to be measured as the dependent variable. The chapter contains three sections. First, we present information on how to identify, operationally define, and understand the target behaviour by using either a neurorehabilitation approach and/or a functional behaviour assessment. The second section focuses on methods to measure the target behaviour in precise and measurable terms. A broad range of methods is discussed, including direct continuous measurement, interval measurement and indirect assessment. The final section of the chapter covers data integrity and reliability. We describe the psychometric features of standardised outcome measures and what needs to be done in their application to single-case methodology. A focus of this section of the chapter is on how to establish the inter-observer agreement on measures that are tailor-made for the case at hand.