Muscle satellite cells are a self-renewing pool of stem cells that give rise to daughter myogenic precursor cells in adult skeletal muscle, where they function in postnatal tissue growth and regeneration. Adult skeletal muscle also contains a novel stem cell population purified as a side population (SP), which actively excludes Hoechst 33342 dye. Muscle SP cells that express the hematopoietic stem cell marker Sca-1 possess the ability to differentiate into hematopoietic cells, skeletal muscle, and satellite cells following transplantation. The muscle SP fraction also contains cells expressing the hematopoietic marker CD45 that are capable of differentiation into hematopoietic cells and muscle cells. Thus, these novel muscle stem cells appear to have characteristics similar to those of hematopoietic stem cells, and can participate in muscle regeneration. This review outlines recent findings regarding different stem cell populations in skeletal muscle, and discusses their involvement in muscle regeneration.