This thesis includes three interconnected essays which, building on the work by Hart and Zingales (2011), lay down the foundations for a new theory of financial intermediation. The first essay explains the Hart and Zingales (HZ) framework and shows that their results are not general. In the HZ model, there is a lack of simultaneous double coincidence of wants, and future income is not pledgeable. This implies that agents need money to trade. However, holding money entails an opportunity cost that leads to a waste of resources. Because of this inefficiency, pecuniary externalities have welfare consequences that private price-taking agents fail to internalize. I find that HZ's result, whereby the market produces inefficiently high levels of l...