The concept of social capital - the networks, norms, and reciprocal trust that enable collective action - has proven irresistible to social scientists seeking to explain why some communities prosper while others stagnate. In its most influential formulation, associated with studies of regional government performance, dense webs of voluntary association are said to cultivate habits of cooperation that spill over into political and economic life; where citizens bowl in leagues, join choral societies, and staff the local parent-teacher council, government works better and markets clear more efficiently. The thesis is elegant, and its policy appeal obvious: if civic participation can be nurtured, prosperity may follow. Yet the very features that make social capital attractive as an explanation render it treacherous as a variable. First, the concept is frequently operationalized by its presumed effects. Analysts infer the presence of trust from the outcomes trust is supposed to produce - efficient institutions, low corruption - and then invoke that trust to explain those same outcomes. The reasoning is circular unless social capital can be measured independently of what it purportedly causes. Second, aggregate measures obscure the distributional question of whose networks count. A neighborhood may be thick with association and yet channel its cooperative energies toward excluding outsiders; the same bonds that lubricate exchange among members can raise the transaction costs borne by everyone else. Robustly measured trust, in other words, may index not a public good but a private cartel. Critics have pressed a further, causal objection. Even granting that associational density and good governance travel together, the direction of influence is unsettled. It is at least as plausible that competent, impartial institutions generate interpersonal trust as that trust generates good institutions: citizens who can rely on courts to enforce contracts and on bureaucracies to treat them fairly have less reason to hoard suspicion. On this reading, civic vibrancy is a symptom of institutional health rather than its cause, and exhortations to join more clubs mistake the effect for the remedy. None of this warrants discarding the concept. The correlation between civic life and institutional performance is among the more durable findings in comparative politics, and the maturation of a research program is often marked precisely by the discovery that its central term was doing too much work. What the objections establish is more modest and more useful: that social capital is best treated not as a self-evident cause but as a placeholder for mechanisms that must be specified and measured one by one. When investigators disaggregate the concept - distinguishing generalized trust from particularized trust, bridging ties from bonding ties, and civic norms from mere associational headcounts - the tidy story fractures into a set of narrower, testable claims, some of which survive scrutiny and some of which do not. That fracturing is not a failure of the field but its proper work. A concept that explains everything explains nothing in particular; the value of social capital may lie less in the answers it supplies than in the sharper questions its difficulties compel us to ask.