This book deals with the human spine with particular emphasis on the lumbar spine. Human gait is traditionally believed to be the exclusive function of the legs. The book presents arguments and data that challenge that belief. It proposes that the spine is the primary engine that makes us move. This engine, inherited from our fish ancestors, was never transferred to any extremities during our long evolutionary journey. From the theoretical and clinical analysis of the normal function of the spine, one can describe the changes that degrade its function. This forces us to define normality in terms of functionality, rather than anatomical perfection. A procedure for the objective measure of spinal function is proposed, and its clinical use discussed. A 35 minutes movie on the function of the spine in both MOV and WMV formats is included.