The primary purpose of this study is to determine the effectiveness of two psychological techniques in chronic low back pain patients treated in hospital pain clinics. The techniques are: cognitive behavioral treatment and mindfulness based stress reduction (MBSR). This is a multiple site project in which patients come from 16 different pain clinics throughout Spain. The control group is formed by patients under usual medical treatment at pain clinics. The two intervention groups also receive usual medical treatment at pain clinics plus one psychological therapy treatment. The sample size is 330 subjects.