Four important domains directly related to low back pain are: pain intensity, low-back-pain-specific disability, patient satisfaction with treatment outcome, and work disability. Within each of the domains, different questionnaires have been proposed. This chapter focuses on validated and widely used questionnaires. Details of the background and the measurement properties, and of the minimally clinically important change (MCIC) using these questionnaires, are described. The MCIC can be estimated using various methods and there is no consensus in the literature on what the most appropriate technique is. This chapter focuses primarily on two adequate and frequently used methods for estimating the MCIC. We argue that the MCIC should not be considered as a fixed value and that the MCIC values presented in this chapter are used as indications. For patients with subacute or chronic low back pain, the MCIC for pain on a visual analogue scale (VAS) should at least be 20 mm and for acute low back pain it seems reasonable to suggest that the MCIC should at least be at the level of approximately 35 mm. If a numerical rating scale (NRS) is used it seems reasonable to suggest that the MCIC should at least be 3.5 and 2.5 for patients with acute and chronic low back pain, respectively. For functional disability as measured with the Roland Disability Questionnaire it seems reasonable that the MCIC should at least be 3.5 points, whereas an MCIC of at least 10 points when the Oswestry Disability Index is used. For global perceived effect, we argue that the MCIC is most appropriately defined in terms of at least 'much improved' or 'very satisfied', instead of including 'slightly improved'. Finally, we argue that, from the point of view of cost effectiveness, every day of earlier return to work is important. The exact value for the MCIC can be determined, taking into account the aim of the measurement, the initial scores, the target population and the method used to assess MCIC.