How does the collective identification of indigenous peoples who span contemporary state borders align with and diverge from those borders? This article analyzes how the Mam, an indigenous people divided by the Guatemala-Mexico border, identify collectively. We further existing sociological literature on collective identity "boundary work" by demonstrating how it is shaped by spatial, and not just symbolic, boundaries. Mam individuals and organizations define symbolic boundaries that sustain political-administrative borders (such as municipal divisions within Guatemala and Guatemala's border with Mexico) in some cases and conflict with them in others. We suggest that state borders and collective identification boundaries become incongruous and contested as social contexts shift and conclude that the symbolic struggle of how to identify as a collectivity has material, and potentially spatial, consequences. K E Y W O R D S : boundary work; collective identities; indigenous peoples; Mam; cross-border nations. Social scientists frequently tie societies, cultures, and nations to specific states, failing to articulate conflicting boundaries between some nations and states (Gupta and Ferguson 1992; Lamont and Moln ar 2002; Rosaldo 1989). In fact, many indigenous nations are fragmented-geographically, socially, culturally, and politically-by state borders. Indeed, conceptualizing peoples as nations bounded within state borders is especially problematic for indigenous nations that span state borders (such as the Mapuche across the Chile-Argentina border, the Tohono O'odham across the Mexico-U.S. border, and the Maya-Mam across the Guatemala-Mexico border). These peoples can be characterized as "cross-border nations" (Warren 2013). Struggles to gain recognition as cross-border nations do not typically involve a demand for statehood. Rather, they entail seeking collective rights such as territory and self-determination, consultation and informed consent (on the part of governments from both countries) about projects that will impact their territory and the natural resources therein, and the ability to develop and maintain This research was partially supported by grants from The Latin American and Caribbean Studies Institute and The Graduate School at The University of Georgia. The authors thank the participants, the leaders of Mam councils in Guatemala and Mexico, A ıda Hern andez, and Ajb'ee Jiménez for helping make this research possible. The authors also thank Pablo Lapegna, Rebecca Hanson, and the anonymous reviewers for their insightful comments on previous drafts.