The shadow banking system is a term for the collection of non-bank financial intermediaries (NBFIs) that legally provide services similar to traditional commercial banks but outside normal banking regulations. S&P Global estimates that, at end-2022, shadow banking held about $63 trillion in financial assets in major jurisdictions around the world, representing 78% of global GDP, up from $28 trillion and 68% of global GDP in 2009.

Examples of NBFIs include hedge funds, insurance firms, pawn shops, cashier's check issuers, check cashing locations, payday lending, currency exchanges, and microloan organizations. The phrase "shadow banking" is regarded by some as pejorative, and the term "market-based finance" has been proposed as an alternative.

Former US Federal Reserve Chair Ben Bernanke provided the following definition in November 2013:

"Shadow banking, as usually defined, comprises a diverse set of institutions and markets that, collectively, carry out traditional banking functions—but do so outside, or in ways only loosely linked to, the traditional system of regulated depository institutions. Examples of important components of the shadow banking system include securitization vehicles, asset-backed commercial paper [ABCP] conduits, money market funds, markets for repurchase agreements, investment banks, and mortgage companies"

Shadow banking has grown in importance to rival traditional depository banking, and was a factor in the subprime mortgage crisis of 2007–2008 and the global recession that followed.

Paul McCulley of investment management firm PIMCO coined the term "shadow banking". Shadow banking is sometimes said to include entities such as hedge funds, money market funds, structured investment vehicles (SIV), "credit investment funds, exchange-traded funds, credit hedge funds, private equity funds, securities broker-dealers, credit insurance providers, securitization and finance companies." Still, the meaning and scope of shadow banking are disputed in academic literature. In China, shadow banking activities are closely associated with commercial banks, involving trust loans, entrusted loans, undiscounted bank acceptance bills, financial products, and interbank business. These activities are often regulated by a weak level and a limited coverage of supervision compared to highly regulated on-balance sheet activities.

According to Hervé Hannoun, deputy general manager of the Bank for International Settlements (BIS), investment banks and commercial banks may conduct much of their business in the shadow banking system (SBS) although most are not classified as SBS institutions themselves. At least one financial regulatory expert has said that regulated banking organizations are the largest shadow banks.

The core activities of investment banks are subject to regulation and monitoring by central banks and other government institutions – but it has been common practice for investment banks to conduct many of their transactions in ways that do not show up on their conventional balance sheet accounting and so are not visible to regulators or unsophisticated investors. For example, before the 2008 financial crisis, investment banks financed mortgages through off-balance-sheet (OBS) securitizations (e.g., asset-backed commercial paper programs) and hedged risk through off-balance sheet credit default swaps. Before the 2008 financial crisis, major investment banks were subject to considerably less stringent regulation than depository banks. In 2008, investment banks Morgan Stanley and Goldman Sachs became bank holding companies, Merrill Lynch and Bear Stearns were acquired by bank holding companies, and Lehman Brothers declared bankruptcy, essentially bringing the largest investment banks into the regulated depository sphere.