Underground money houses are a common name for a kind of organizations engaged in underground illegal financial business, which means that criminals engage in illegal and criminal activities such as cross-border remittance, buying and selling foreign exchange, payment and settlement of funds without the approval of the competent state department for the purpose of illegal profits, and are generally punished as illegal business. According to different regions, underground banks are engaged in many illegal businesses, such as illegally absorbing public deposits, illegally borrowing and lending, illegally lending at high interest rates, illegally buying and selling foreign exchange, illegally pawning, private equity funds and so on. Among them, illegal trading in foreign exchange and being used as a money laundering tool are the most well known. According to Article 3 of the Measures for Banning Illegal Financial Institutions and Illegal Financial Business Activities promulgated and implemented in the State Council on June 3, 1998, "Illegal financial institutions refer to institutions that are established without the approval of the People's Bank of China to engage in or engage in financial business activities such as taking deposits, issuing loans, financing guarantees, and foreign exchange trading." Therefore, underground banks are illegal financial institutions.

Underground money houses also have many manifestations abroad. In the United States, Canada, Japan and other places, the Chinese community is called "underground bank", which mainly engages in remittance and collection business of Chinese in the community. There are many organizations similar to underground banks in Asia, and some underground banks have developed into networked and specialized underground banking systems in India and Pakistan.