During the last decades, CSOs in Mexico have not only substantially grown in number, but have also acquired an important role in policy making. CSOs have become more institutionally consolidated and some have professionalized their work, becoming an important alternative for growing sectors of activists and technicians who offer concrete support services to grassroots development initiatives. Moreover, CSOs are no longer focused on developmental projects but have moved to a political arena, where they are carrying out other functions such as monitoring elections, protecting human rights, designing and implementing social and developmental projects, giving voice to excluded minorities. The goal of this chapter is twofold: on the one hand, to examine to what extent some CSOs in Mexico conduct policy analysis, as well as the sources of the growing demand for policy research by the third sector. On the other, to explore if they use policy analysis to exert influence in policy-making, and the factors promoting and constraining their impact in shaping Mexican public policy.