With the present review of literature, the authors intended to compare the definition terms, selection criteria, and measurement tools or methods used in different studies related to chronic low back pain (CLBP) patients. The relevance of including all the above information is that any health-care professional can use them to evaluate their treatment methods with CLBP patients or use them in study designs according to their objectives. These measurements concern pain measurements, measures that were used to describe the CLBP pain, questionnaires used to measure the CLBP patients' responses to pain, the pain effects on their living activities, and also measurements of the physical abilities and functional performance. A computerized literature search in English MEDLINE was conducted using "low back pain," and "flexibility, "strength,""evaluation,""functional level," and "measurements" as search words. Identified abstracts were scanned, and useful articles were acquired for further review. Interms of CLBP definitions, the authors concluded that is best defined as a lumbar, sacral, or lumbosacral spinal pain that is continuous or essentially continuous but low level punctuated by exacerbations of pain, each of which is characterized as "acute." In order to establish the criteria for selecting participants in a study design related to CLBP, pain characteristics and clinical diagnoses have to be taken into consideration for obtaining homogeneity of groups. Finally, the selection of measurement tools and evaluation methods is related to the study's goals, the specialization of the researchers, and their validity.