This study attempts to integrate and contribute to the literature on Karl Marx's ideas about credit, interest, and financial instability. The first section focuses on Marx's analysis of the forms and functions of credit. The second focuses on Marx's ideas about the nature of interest and his theory of the determination of the interest rate. The third discusses Marx's analysis of the role the financial system plays in intensifying business cycles and business crisis. Ultimately, Marx's analysis demonstrates that the financial system, while essential for capitalist development, encourages processes that continually exacerbate the inherent instability of a capitalist economy.