Functional ability is an important outcome measure in people with chronic low back pain (CLBP). It is predominantly measured using self-report and physical performance testing. Both methods have limitations, and neither provides an objective insight into a person's functional ability in everyday life. The purpose of this thesis was to investigate if free-living physical activity (PA) monitoring could be used as an objective marker of functional ability during everyday life in people with chronic low back pain (CLBP).