The Neotropics is one of the most biodiverse regions of the world. However, understanding the historical processes underlying this diversity is a challenging task due in part to the complex geographic and climatological history of this region. For mammals, this task is further hindered by our lack of fundamental knowledge about biodiversity, such as the number of species occurring in this area, their distribution, and how they are related to each other. This is particularly true for a group of small opossums (Thylamyini), which are broadly distributed throughout the Neotropics but are often morphologically indistinguishable from each other. There are 37 currently recognized species of Thylamyini, but preliminary analyses suggest that species diversity may be nearly double that. This project aims to discover the true diversity of this ecologically important group of mammals using multiple nuclear and mitochondrial DNA sequences as well as morphology. This revised view of mammalian diversity will have profound implications for our understanding of diversification in the Neotropics and will provide the basis for much needed conservation strategies in this time of global extinction. In addition, the proposed outreach and educational activities will continue to strengthen developing collaborations between US and Colombian institutions of higher education.