Acupuncture is based on ancient Chinese theories of the flow of chi(energy) through distinct meridians or pathways that cover the body.  According to ancient theory, acupuncture allows chi to flow to areas where it is deficient and away from where it is excess.  In this way, acupuncture regulates and restores the harmonious energetic balance of the body.  In Chinese there is a saying, "there is no pain if there is free flow, if there is pain, there is no free flow."