The purpose of this qualitative inquiry was to build a substantive theory of family support following the systemic design of grounded theory. In particular, the inquiry addressed the behavioral and attitudinal patterns of family support that occur among undocumented Central American immigrant families in the United States. In order to collect data, in-depth interviews were conducted with nine undocumented Central American immigrant families living in a metropolitan area of the Midwest and with three staff members from social institutions involved in assisting undocumented immigrants. These interviews were tape-recorded and transcribed verbatim. The collected data was systematically analyzed using the procedures of open, axial, and selective coding of the systematic design. Ten major categories and 41 subcategories were identified through this multiple coding. Family support, which emerged as the core category, was used to link all the emergent categories and subcategories. An organizational scheme, which replicates the grounded theory paradigm, visually depicts the multiple links that were found. A set of 20 propositions and 17 subpropositions, which identified the relationships that occur among the main concepts of the theory, were also developed in order to test, refine, and expand the theory through further research. The resulting theory of family support offers instrumental information for policy makers, clinicians, and clergy.