  This Chapter reviews statistical models for the probability distribution of
money developed in the econophysics literature since the late 1990s. In these
models, economic transactions are modeled as random transfers of money between
the agents in payment for goods and services. Starting from the initially equal
distribution of money, the system spontaneously develops a highly unequal
distribution of money analogous to the Boltzmann-Gibbs distribution of energy
in physics. Boundary conditions are crucial for achieving a stationary
distribution. When debt is permitted, it destabilizes the system, unless some
sort of limit is imposed on maximal debt.
