We use the MSRB Transaction Reporting System audit trail to study dealer intermediation and liquidity provision in decentralized over-the-counter markets. The dealership network in municipal bonds exhibits a hierarchical core-periphery structure with about 20-30 highly interconnected dealers at its core and several hundred peripheral dealer firms. Market quality varies significantly across dealers depending on their interconnectedness and centrality within the trading network. Central dealers charge larger trading costs to investors and face lower loss probabilities than peripheral dealers. Yet, investor orders flow through central dealers. Central dealers place bonds more readily with investors than other dealers, consistent with smaller search frictions. Central dealers also provide more liquidity immediacy than peripheral dealers, leading central dealers to hold larger and more volatile inventories, keep bonds longer, and intermediate fewer pre-arranged trades. Investors trade with central dealers when liquidity is otherwise low. Central dealers can thus be considered liquidity providers of last resort. (Co-author Dan Li)