TransUnion is a consumer credit reporting agency. TransUnion collects and aggregates information on over one billion individual consumers in over thirty countries including "200 million files profiling nearly every credit-active consumer in the United States". Its customers include over 65,000 businesses. Based in Chicago, Illinois, TransUnion's 2014 revenue was US$1.3 billion. It is the smallest of the three largest credit agencies, along with Experian and Equifax (known as the “Big Three”).

TransUnion also markets credit reports and other credit and fraud-protection products directly to consumers. Like all credit reporting agencies, the company is required by US law to provide consumers with one free credit report every year.

TransUnion was originally formed in 1968 as a holding company for the railroad leasing organization, Union Tank Car Company. The following year, it acquired the Credit Bureau of Cook County, which possessed and maintained 3.6 million card files. In 1981, a Chicago-based holding company The Marmon Group acquired TransUnion for approximately $688 million. Almost thirty years later, in 2010, Goldman Sachs Capital Partners and Advent International acquired it from Madison Dearborn Partners In 2014, TransUnion acquired Hank Asher’s data company TLO. On June 25, 2015, TransUnion became a publicly traded company for the first time, trading under the symbol TRU.

TransUnion has evolved its business over the years to offer products and services for both businesses and consumers. For businesses, TransUnion has evolved its traditional credit score offering to include trended data that helps predict consumer repayment and debt behavior. This product, referred to as CreditVision, launched in Oct. 2013.

Its SmartMove™ service facilitates credit and background checks for consumers who may be serving in a landlord capacity.

In September 2013, the company acquired eScan Data Systems of Austin to provide post-service eligibility determination support to hospitals and healthcare systems. The technology was integrated into TransUnion’s ClearIQ platform that tracks patients demographic and insurance related information to support benefit verification.

In November 2013, TransUnion acquired TLO LLC, a company that leverages data in support of its investigative and risk management tools. Its TLOxp technology aggregates data sets and using a proprietary algorithm to uncover relationships between data that were not possible before.

In 2014, a TransUnion analysis found that reporting rental payment information to credit bureaus can positively affect credit scores. To benefit consumers, they initiated a service called ResidentCredit, making it easy for property owners to report data about their tenants to TransUnion on a monthly basis. These reports include the amount each tenant pays, the timeliness of their last payment, and any remaining balance the tenant currently owes. As a result, companies have started reporting rent payment information to TransUnion, including YapStone, Inc., Pangea Real Estate, and RentReporters.

As part of its fraud protection products, it also offers business a tool called DecisionEdge that aggregates the data needed to prevent fraud through a system that customizes the information needed to finalize a transaction.

For consumers, TransUnion offers credit monitoring and identity theft protection tools. The company’s app offers a function called CreditLock that allows an individual to unlock and lock their credit to help protect against fraudulent activity.

In 2003, Judy Thomas of Klamath Falls, Oregon, was awarded $5.3 million in a successful lawsuit against TransUnion. The award was made on the grounds that it took her six years to get TransUnion to remove incorrect information in her credit report.

In 2006, after spending two years trying to correct erroneous credit information that resulted from being a victim of identity theft, a fraud victim named Sloan filed suit against all three of the USA's largest credit agencies. TransUnion and Experian settled out of court for an undisclosed amount. In Sloan v. Equifax, a jury awarded Sloan $351,000. "She wrote letters. She called them. They saw the problem. They just didn't fix it," said her attorney, A. Hugo Blankingship III.

TransUnion has also been criticized for concealing charges. Many users complained of not being aware of a $17.95/month charge for holding a TransUnion account.

In March 2015, following a settlement with the New York Attorney General, TransUnion, along with other credit reporting companies, Experian and Equifax, agreed to help consumers with errors and red flags on credit reports. Under the new settlement, credit-reporting firms are required to use trained employees to respond when a consumer flags a mistake on their file. These employees are responsible for communicating with the lender and resolving the dispute.

In June 2017, a California jury ruled against Transunion with a $60 Million dollar verdict in the largest FCRA verdict in history. The San Francisco federal court jury awarded $60 million in damages to consumers who were falsely reported on a government list of terrorists and other security threats. The plaintiffs' team of attorneys at Francis & Mailman, P.C. partnered with another California based firm in the class action.

On 13 October 2017, the website for TransUnion's Central American division was reported to have been redirecting visitors to websites that attempted drive-by downloads of malware disguised as Adobe Flash updates. The attack had been performed by hijacking third-party analytics JavaScript from Digital River brand FireClick.