Disagreements over the systemic implications-the future-of financialization can be traced in part to the absence of sustained attention to the role of banking firms in driving this secular shift forward. That is, the financialization literature lacks an adequate microfoundation. Accounting for the drivers of financialization processes solely at the macro level overlooks the problems of how these processes came about and whether they are sustainable. This paper addresses this explanatory gap, arguing that a key independent microeconomic driver of increasing financialization did exist: the incessant efforts by money-centre banks in the USA to break out of Depression-era restrictions on their size, activities, and markets. These banks' growth strategies in turbulent times led to an institutional (meso) shift-the rise of a megabank-centred shadow banking system-that now shapes global financial architecture even while operating in ways that are unsustainable. In short, too-big-to-manage megabanks are at the heart of the fragility and instability of the economy today.