This document analyzes how different measures of market concentration (the Herfindahl-Hirschman Index and the Dominance Index) explain investment decisions of Mexican manufacturing firms. The authors estimate these concentration measures at the subsector level using firm-level data from Mexico's 2003 Economic Census. They then use a log-linear model to examine the relationship between investment and these concentration measures, controlling for factors like investment opportunities, cash flow, firm size, and capital intensity. The results suggest that the Dominance Index may better explain investment decisions of Mexican firms compared to the Herfindahl-Hirschman Index. Specifically, the conclusions indicate that higher market concentration is associated with reduced investment.