Heart failure is one of the most important cardiovascular diseases, with high mortality, and invasive treatment such as mechanical circulatory support and cardiac transplantation is sometimes required for severe heart failure. Therefore, the development of less invasive and more effective therapeutic strategies is desired. Cell therapy is attracting growing interest as a new approach for the treatment of heart failure. As a cell source, various kinds of stem/progenitor cells such as bone marrow cells, endothelial progenitor cells, mesenchymal stem cells (MSC) and cardiac stem cells have been investigated for their efficacy and safety. Especially, bone marrow-derived MSC possess multipotency and can be easily expanded in culture, and are thus an attractive therapeutic tool for heart failure. Recent studies have revealed the underlying mechanisms of MSC in cardiac repair: MSC not only differentiate into specific cell types such as cardiomyocytes and vascular endothelial cells, but also secrete a variety of paracrine angiogenic and cytoprotective factors. It has also been suggested that endogenous MSC as well as exogenously transplanted MSC migrate and participate in cardiac repair. Based on these findings, several clinical trials have just been started to evaluate the safety and efficacy of MSC for the treatment of heart failure.