Background context: Modern scientific investigations into spinal manipulative therapy (SMT) began in 1975. Conditions often treated include acute and chronic low back pain, radicular pain, neck pain, and some forms of headache. The field of spinal manipulation has often been treated by the literature, incorrectly, as being homogeneous. Much of the confusion regarding this form of treatment can be traced to the ambiguity surrounding the procedures themselves. This report summarizes the clinical biomechanics of SMT and evidence for its associated manipulable lesion is reviewed. Finally, a classification system based on biomechanics is proposed that may facilitate more detailed research in the future. Purpose: A categorization system for SMT was sought that would be more objective than is clinically available. Such a system may serve as a means to strengthen future studies, determine operating principles, applicability, treatment effectiveness, and nature of the manipulable lesion. Study design: Literature synthesis. Methods: A search of the indexed biomechanical and medical literature as well as a hand search of published works was conducted. The criteria for article selection consisted of studies that included measurements of mechanical characteristics of treatment techniques used under the general headings of SMT or manual therapy. A second set of studies was identified that explored the biomechanics of buckling behavior of vertebral segments as a model of the manipulable lesion. Quantitative characteristics of SMT were extracted and grouped to form a basis for classification. Results: A total of 31 articles were identified that contained quantitative data on the biomechanical properties of SMT methods. An additional seven studies were found that quantified spinal buckling behavior. Common features of SMT procedures lead to a matrix that biomechanically characterizes the types of procedures in use. Buckling behavior was compared qualitatively with clinical observations to form a plausible and evidence-based hypothesis of the manipulable lesion. Conclusions: There currently are a number of named systems of manual procedures. No current triage system is available that predicts which patient has the greater likelihood of benefiting from manual treatment or the procedure type. The biomechanical parameters of SMT form a systematic characterization of manual procedures. Such a system may be used in future studies to test hypotheses of treatment effect from quantitatively defined procedures.