This article discusses various hypotheses relating to the origin and operation of business groups in Mexico, and it proposes a model to explain the sources of their total asset growth. It highlights their growing contribution to Mexican gdp, but notes that their shares of employment and profits are smaller. Over time, sales and assets have clearly tended to become more concentrated in the largest groups. The paper concludes that the main financing sources for asset growth between 2005 and 2007 were firstly debt and secondly capital contributions from shareholders. It also finds that the leading groups invest discretely over time and tend to "overinvest" to block the entry of other competitors.