Classification systems for patients with low back pain have become more abundant in the literature since the mid-1980s. Some classification systems are designed to determine the most appropriate treatment, some are designed to aid in prognosis, and others are designed to identify pathology. Still other classification systems categorize patients into homogeneous groups based on selected variables. The purpose of this review is to describe and critically evaluate low back pain classification systems. Several classification systems were summarized and examined. Four classification systems that were judged to be the most commonly cited and most relevant to physical therapists were critiqued using a more thorough systematic approach. The analysis suggests that future research should address the usefulness of existing classification systems as well as the development of new classification systems designed using commonly accepted measurement principles.