An extraordinary credit boom that created many first-time homeowners and financed a wave of corporate takeovers seems to be waning. Home buyers with poor credit are having trouble borrowing. Institutional investors from Milwaukee to Düsseldorf to Sydney are reporting losses. Banks are stuck with corporate debt that investors won't buy. Stocks are on a roller coaster, with financial powerhouses like Bear Stearns Cos. and Blackstone Group coming under intense pressure.

The origins of the boom and this unfolding reversal...