To properly advise a client, the tax practitioner needs a basic understanding of how interest is imposed and computed on both underpayments and overpayments. Interest calculations in most commercial applications is straight-forward. As tax law and tax practice have evolved and become more complex, so have the interest provisions in the Internal Revenue Code. The IRS may make errors in its calculations or have insufficient information to compute interest accurately; in addition, there are some disputed issues or areas where the law is uncertain. For large corporations or high-income taxpayers, disputes can involve millions of dollars. Some errors can even affect low-income taxpayers with seemingly simple facts.