Résumé : The microfinance industry has been celebrated both for its social impact on poverty alleviation and for its profitability. With issues of over-indebtedness emerging among microfinance customers, both achievements are at risk. This paper contributes to the industry's understanding of the definition and causes of over-indebtedness. It reveals why the 5 myths of microfinance over-indebtedness erroneously oversimplify the reality of microfinance customers. The paper works with theoretical and empirical contributions from economics, psychology and sociology, and unites microfinance specific findings with the general consumer finance literature. In addition to external influences, it highlights the responsibility of lenders in driving microfinance customers into over-indebtedness. It also recognises the role that borrowers involuntarily play in over-indebting themselves. Enhancing our understanding of what microfinance over-indebtedness is and how it is caused, the paper provides the basis for tailoring over-indebtedness solutions to the root causes of the phenomenon and addressing the challenge at all suitable levels. Latest version (June 2011) : http://www.solvay.edu/sites/upload/files/CEB_WorkingPapers/LastUpdate/wp10048.pdf