This thesis presents a set of studies which investigated chronic low back pain. The specific aim of this thesis was to develop reliable methods for the assessment and rehabilitation of chronic low back pain. The two assessment methods tested were a broad based clinical evaluation and an isokinetic assessment of trunk muscle strength. The first part of the thesis describes the reliability and validity studies of the clinical methods for measuring trunk mobility, trunk muscle strength, spinal shape and palpation. A total of 27 physical tests were studied using 70 patients and 10 normal subjects. Twenty-three of these tests were found to be reliable and were incorporated into the isokinetic assessment study. The second part of the study reports the standardisation and reliability studies for the isokinetic assessment of trunk muscle strength in 70 normal subjects and 120 patients with chronic low back pain. The results showed that the main isokinetic measures were reliable for both normal subjects and patients. There was a significant learning effect from test 1 to test 2 in both normal subjects and patients. The magnitude of this learning effect was greater in patients than normal subjects. The normal subjects were followed up by a postal questionnaire in a two year prospective study to predict future back pain using the isokinetic measures. None of the measures used showed any significant differences between those subjects who developed back pain and those who did not. The third part of the thesis describes studies to develop a rehabilitation programme for patients with chronic low back pain using the isokinetic machines, both as a means of monitoring progress and as an exercise regime. The first study of 26 patients indicated that the programme was safe and effective, but also revealed a major problem with adherence to a six week exercise programme. The second study investigated the problem of adherence and examined the time course of response to isokinetic exercise by repeating the tests at three weeks and six weeks.