examination of the records and reports of an enterprise by specialists other than those responsible for their preparation. Public auditing by independent, impartial accountants has acquired professional status and become increasingly common with the rise of large business units and the separation of ownership from managerial control. The public accountant performs tests to determine whether the management's statements were prepared in accord with generally accepted accounting principles and fairly present the firm's financial position and operating results; such independent evaluations of management reports are of interest to actual and prospective shareholders, bankers, suppliers, lessors, and government agencies.

In English-speaking countries, public auditors are usually certified, and high standards are encouraged by professional societies. Most European and Commonwealth nations follow the example of the United Kingdom, where government-chartered organizations of accountants have developed their own admission standards. Other countries follow the pattern in the United States, where the states have set legal requirements for licensing. Most national governments have specific agencies or departments charged with the auditing of their public accounts—e.g., the General Accounting Office in the United States and the Court of Accounts (Cour des Comptes) in France.

Internal auditing, designed to evaluate the effectiveness of a company (corporation)'s accounting system, is relatively new. Perhaps the most familiar type of auditing is the administrative audit, or pre-audit, in which individual vouchers, invoices, or other documents are investigated for accuracy and proper authorization before they are paid or entered in the books.

In addition, the assurance services of professionally certified accountants include all of the following: financial, compliance, and assurance audits; less-formal review of financial information; attestation about the reliability of another party's written assertion; and other assurance services not strictly requiring formal audits (e.g., forward-looking information and quality assertions).

Historians of accounting have noted biblical references to common auditing practices, such as dual custody of assets and segregation of duties, among others. In addition, there is evidence that the government accounting system in China during the Zhao dynasty (1122–256 BC) included audits of official departments. As early as the 5th and 4th centuries BC, both the Romans and Greeks devised careful systems of checks and counterchecks to ensure the accuracy of their reports. In English-speaking countries, records from the Exchequers (Exchequer) of England and Scotland (1130) have provided the earliest written references to auditing.

Despite these early developments, it was not until the late 19th century, with the innovation of the joint-stock company (whose managers were not necessarily the company's owners) and the growth of railroads (with the challenge of transporting and accounting for significant volumes of goods), that auditing became a necessary part of modern business. Since the owners of the corporations were not the ones making the day-to-day business decisions, they demanded assurances that the managers were providing reliable and accurate information. The auditing profession developed to meet this growing need, and in 1892 Lawrence R. Dicksee published A Practical Manual for Auditors, the first textbook on auditing. Audit failures occur from time to time, however, drawing public attention to the practice of accounting and auditing while also leading to a refinement of the standards that guide the audit process.

Given the nature of the audit function, auditors increasingly find themselves subject to legal and other disciplinary sanctions. Unlike other professionals, however, their liability is not limited to the clients who hire them. Auditors are increasingly held liable to third parties, including investors and creditors, who rely on the audited financial statements in making investment decisions.

A company's internal accountants are primarily responsible for preparing financial statements. In contrast, the purpose of the auditor is to express an opinion on the assertions of management found in financial statements. The auditor arrives at an objective opinion by systematically obtaining and evaluating evidence in conformity with professional auditing standards. Audits increase the reliability of financial information and consequently improve the efficiency of capital markets. Auditing standards require that all audits be conducted by persons having adequate technical training. This includes formal education, field experience, and continuing professional training.

In addition, auditors must exhibit an independence in mental attitude. This standard requires auditors to maintain a stance of neutrality toward their clients, and it further implies that auditors must be perceived by the public as being independent. In other words, it mandates independence in fact and in appearance. Thus, any auditor who holds a substantial financial interest in the activities of the client is not seen as independent even if, in fact, the auditor is unbiased.

The issue of auditor independence grew more difficult toward the end of the 20th century, especially as auditing firms began offering nonattestation functions (such as consulting services) to new and existing clients—particularly in the areas of taxation, information systems, and management. While there was no legal reason for preventing accounting firms from extending their business services, the possibilities for a conflict of interest made it increasingly necessary for auditors to indicate the nature of the work performed and their degree of responsibility.

Inaccurate financial reporting can be the result of deliberate misrepresentation, or it can be the result of unintended errors. One of the most egregious recent examples of a financial reporting failure occurred in 1995 in the Singapore office of Barings PLC, a 233-year-old British bank. In this case fraud resulted from a lack of sufficient internal controls at Barings over a five-year period, during which time Nicholas Leeson, a back-office clerk responsible for the accounting and settlement of transactions, was promoted to chief trader at Barings's Singapore office. With his promotion, Leeson enjoyed an unusual degree of independence; he was in the exceptional position of being both chief trader and the employee responsible for settling (ensuring payment for) all his trades, a situation that allowed him to engage in rogue (unauthorized) trades that went undetected. As if to condone Leeson's actions, his managers at Barings had given him access to funds that could cover margin calls (purchases made with borrowed money) for his clients. Although Leeson was losing huge sums of money for the bank, his dual responsibilities allowed him to conceal his losses and to continue trading. When the collapse of the Japanese stock market led to a $1 billion loss for Barings, Leeson's actions were finally discovered. Barings never recovered from the loss, however, and it was acquired by Dutch insurance company ING Groep NV in 1995 (sold again in 2004). Interestingly, in this case internal auditors did warn management about the risk at the Singapore office months before the collapse, but the warnings went unheeded by top executives, and the audit report was ignored.

In 2001 the scandal surrounding the Barings collapse was dwarfed by discoveries of corruption in large American corporations. Enron Corp.—an energy trading firm that had hidden losses in off-the-books partnerships and engaged in predatory pricing schemes—declared bankruptcy in December 2002. Soon after Enron became the subject of a Securities and Exchange Commission (SEC) inquiry, Enron's auditing firm, Arthur Andersen LLP, was also named in an SEC investigation; Arthur Andersen ultimately went out of business in 2002. In roughly the same period, the telecommunications firm WorldCom Inc. used misleading accounting techniques to hide expenses and overstate profits by $11 billion. Instances of accounting fraud uncovered in Europe in the early 21st century included Dutch grocery chain Royal Ahold NV, which in 2003 was found to have overstated profits by roughly $500 million.

In the United States, auditing standards require the auditor to state whether the financial reports are presented in accordance with generally accepted accounting principles (GAAP). Many other countries have adopted the standards supported by the International Accounting Standards Board (IASB) in London. The IASB standards, often less lenient than GAAP, have been increasingly seen as more-effective deterrents to large-scale auditing failures such as those that took place at Enron and WorldCom.

No auditing technique can be foolproof, and misstatements can exist even when auditors apply the appropriate techniques. The auditor's opinion is, after all, based on samples of data. A management team that engages in organized fraud by concealing and falsifying documents may be able to mislead auditors and other users and go undetected. The best any auditor can provide, even under the most-favourable circumstances, is a reasonable assurance of the accuracy of the financial reports.