Assurance service is an independent professional service, typically provided by Chartered or Certified Public Accountants, with the goal of improving the information or the context of the information so that decision makers can make more informed, and presumably better, decisions. Assurance services provide independent and professional opinions that reduce information risk (risk from incorrect information).
Definition and distinction from other services
The technical definition of assurance requires five components set out in the International Framework for Assurance Engagements:
1) A three party relationship- the responsible party who prepares the information to be assured; the independent practitioner who assures the information; and the users who are expected to rely on the information. In the case of an audit, the responsible party are the management of the company, the practitioner is the audit firm and the users are primarily the shareholders.
2) Agreed subject matter - in the case of an audit, this would be the annual accounts of a company. However it could be almost anything in practice - the systems operated by a state lottery, a company's greenhouse gas emissions, controls over a supply chain etc.
3) Suitable criteria - this means that there must be some agreed framework to which the information may be compared. In the case of an audit, this will be the form of company accounts mandated by the appropriate laws, regulations and accounting standards in a particular jurisdiction. For other subject matter, it might be the design and proper operation (as specified) of the lottery systems and controls.
4) Sufficient appropriate evidence. The practitioner must obtain sufficient appropriate evidence that the subject matter information (the data related to the subject matter) agrees with the criteria.
5) A conclusion expressed in a written report.
Audits are therefore a type of assurance service. However, audits only test the validity of the assertions in financial statements, and are subject to regulation under International Standards on Auditing. Assurance engagements designed to test historical financial information are referred to as assurance reviews (these are regulated by International Standard on Review Engagements (ISRE 2400), but assurance reports can be obtained over many other subject matters and will then be subject to ISAE 3000 or other individual Standards on Assurance Engagements.
Consulting services are not considered as assurance  because in consulting services, an accountant generally uses their professional knowledge to make recommendations for a future event or a procedure, such as the design of an information system or accounting control system. In contrast, assurance services are designed to test the validity of past data of the business cycles. Although there is no boundary to what can be tested by assurance services, professional accountants cannot accept any engagement for which they do not believe themselves to be competent.
Agreed upon procedures do not constitute an assurance procedure under the above definition, because no conclusion is given. However, they are often loosely referred to as "assurance". Similarly compilation engagements (which also have no conclusion) are often described as giving assurance, but are not strictly assurance engagements.
Technical guidance for practitioners wishing to undertake assurance services can be found in ISAE (International Standard on Assurance Engagements) 3000 ISAE 3000 and in The Assurance Sourcebook  published by ICAEW (Institute of Chartered Accountants in England and Wales which also includes advice for companies wishing to choose between various assurance services.
- "Welcome to Assurance Services". American Institute of Certified Public Accountants. Archived from the original on 5 December 2006. Retrieved 14 June 2010.
- International Standard on Assurance Engagements 3000 (Revised 2013), paras 6 (c) and A1.
- "SSAE #10 Attest Engagements, AT §101, clause .21, second general standard." (PDF). American Institute of Certified Public Accountants.
- "International Standard on Assurance Engagements (ISAE) 3402, §13(a)(i)" (PDF). International Auditing and Assurance Standards Board.
- "Are Assurance Services Provided by Auditors on Initial Public Offerings Influenced by Market Conditions?" COPLEY, PAUL A.; DOUTHETT JR., EDWARD B. Contemporary Accounting Research Summer2009, Vol. 26 Issue 2, p453-476. 24p.
- "Determinants of the Maximum Level of Assurance for Various Assurance Services" Ruhnke, Klaus; Lubitzsch, Kay. International Journal of Auditing Nov2010, Vol. 14 Issue 3, p233-255. 23p.
- "The Different Types of Assurance Services and Levels of Assurance Provided" Hasan, Mahreen; Maijoor, Steven; Mock, Theodore J.; Roebuck, Peter; Simnett, Roger; Vanstraelen, Ann. International Journal of Auditing Jul2005, Vol. 9 Issue 2, p91-102. 12p.
- "The Relative Importance of Firm Incentives versus Country Factors in the Demand for Assurance Services by Private Entities" FRANCIS, JERE R.; KHURANA, INDER K.; MARTIN, XIUMIN; PEREIRA, RAYNOLDE. Contemporary Accounting Research Summer2011, Vol. 28 Issue 2, p487-516.
- "Assurance Services for Sustainability Reports: Standards and Empirical Evidence" Manetti, Giacomo; Becatti, Lucia. Journal of Business Ethics. Jul2009 Supplement 1, Vol. 87, p289-298.
- "The Competitive Market for Assurance Engagements on Greenhouse Gas Statements: Is There a Role for Assurers from the Accounting Profession?" Huggins, Anna; Green, Wendy J.; Simnett, Roger. Current Issues in Auditing. 2011, Vol. 5 Issue 2, pA1-A12.