Zillow Group, or simply Zillow, is an online real estate database company that was founded in 2006 and created by Rich Barton and Lloyd Frink, former Microsoft executives and founders of Microsoft spin-off Expedia. Spencer Rascoff is the current CEO of Zillow, Inc.

Zillow has stated that it is a media company that generates revenue by selling advertising on its web site. In April 2009, Zillow announced a partnership to lend its real estate search engine to the web sites of more than 180 United States newspapers as a part of the Zillow Newspaper Consortium. Zillow shares advertising revenue from the co-branded sites with the newspapers and extends its reach into local markets.

In February 2011, Zillow and Yahoo! Real Estate launched an exclusive partnership creating the largest real-estate advertising network on the web, according to comScore Media Metrix.

Zillow has data on 110 million homes across the United States, not just those homes currently for sale. In addition to giving value estimates of homes, it offers several features including value changes of each home in a given time frame (such as one, five, or 10 years), aerial views of homes, and prices of comparable homes in the area. Where it can access appropriate public data, it also provides basic information on a given home, such as square footage and the number of bedrooms and bathrooms. Users can also get current estimates of homes if there was a significant change made, such as a recently remodeled kitchen. Zillow provides an application programming interface (API) and developer support network.

In December 2006, Zillow launched three new pieces of functionality: allowing users to post homes for sale and set a "Make Me Move" price (an informal way to pre-market a home), as well as a real estate wiki. In 2006, Zillow teamed with Microsoft to offer Bird's Eye View, a feature in Microsoft Virtual Earth, that shows (in certain areas) clearer aerial photographs taken from airplanes rather than conventional satellite imagery. Zillow uses this functionality for entertainment-focused features on famous homes.

In December 2009, Zillow expanded its services to include the rental market. The addition of rental listings enabled users to list a home for rent and search for both rental homes and homes for sale.

In late 2013, Zillow began powering AOL Real Estate. In July 2014, Zillow also took over the real estate portal for MSN Real Estate.In October 2017, Zillow announced to add 3D tours to get 360-degree photos of houses they're interested in buying or renting.

On April 3, 2008, Zillow launched a service called Zillow Mortgage Marketplace. This service allows for borrowers to get custom loan quotes without revealing personally identifying information.

Zillow Mobile apps allow users to view nearby homes based on the user's location.

On December 16, 2008, Zillow launched Zillow Advice, allowing people to ask real estate questions online and get answers from the Web site's community of experts.

Zillow produces home value reports for the nation and over 130 metropolitan statistical areas. The reports identify market trends including, but not limited to: five and 10-year annualized change, negative equity, short sales and foreclosure transactions.

Zillow also releases a Homeowner Confidence Survey. The survey is conducted by Harris Interactive and measures homeowners' perceptions about home value changes of their own home and the local market.

On August 26, 2016 the Bellingham Herald published housing prices and rental statistics obtained from Zillow which said that these statistics conflicted with those obtained from the Housing Finance Agency.

The Zillow data team has created a database of nearly 7,544 neighborhood boundaries in the largest cities in the U.S. and made them available via Creative Commons Attribute-Sharealike license.

The company said it had more than 24 million unique visitors in September 2011, representing year-over-year growth of 103 percent. Of those users, 90% own a home and more than three quarters are looking to buy or sell within the next two years, helping others to buy or sell or looking to rent. Zillow claims over 50 million U.S. homes have been viewed. In some cities more than 90% of all homes that exist have been viewed, including San Francisco, Los Angeles, Boston and Seattle.

Zillow determines an estimate ("Zestimate," pronounced "ZEST-imate") for a home based on a range of publicly available information, including public data and sales of comparable houses in a neighborhood. According to Zillow, the Zestimate is a starting point in determining a home's value. The accuracy of the Zestimate varies by location depending on how much information is publicly available, but Zillow allows users to check the accuracy of Zestimates in their own region against actual sales. In many U.S. states, information on the transfer prices of real estate is readily available and accessible by the general public and is not exclusively held in realtors' databases.. Zillow also supplies Zestimate Accuracy ratings for varies cities on its website.

In March 2011, Zillow released Rent Zestimates, which provide estimated rent prices for 90 million homes on both the Zillow website and all of its mobile apps.

On June 14, 2011, Zillow changed its algorithm used to calculate Zestimates. In addition to changing the current Zestimate for millions of homes throughout the country, Zillow changed historical Zestimate value information dating back to 2006.

In 2007, The Wall Street Journal studied the accuracy of Zillow's estimates and found that they "often are very good, frequently within a few percentage points of the actual price paid. But when Zillow is bad, it can be terrible."

In October 2006, the National Community Reinvestment Coalition filed a complaint with the Federal Trade Commission stating that Zillow was "intentionally misleading consumers and real-estate professionals to rely upon the accuracy of its valuation services despite the full knowledge of the company officials that their valuation Automated Valuation Model (AVM) mechanism is highly inaccurate and misleading." In a letter dated May 4, 2007, the FTC elected not to investigate this complaint, which was later withdrawn by the NCRC.

Using data published on the Zillow website, the typical Zestimate error in the United States in July 2016 was $14,000.

While factors contributing to estimates are described elsewhere, Zillow seemingly overemphasises home square footage as the major metric driving property valuation. This method may not be unique to Zillow, but unduly distorts value expectations. Listings in areas where land is priced at high premiums often reflect an identical Zillow Estimate to that of nearby homes with comparable interior square footage, but where the home might be decades older. Condition, age of home, special features, proximity to nuisances are insufficiently factored into the estimate. Zillow has made some effort to add balance by including option for owners to provide their own value estimate; but, these figures can be similarly unreliable as being opinion instead of quantifiable.

In 2014, Zillow faced several lawsuits from former employees at the Zillow operation in Irvine, California, alleging violations of California Labor Code and California Business and Professions Code. On February 26, 2016, the U.S. District Court for the Central District of California certified the class to include anyone who worked as an inside sales consultant at Zillow between November 2010 and January 2015. Among the numerous allegations brought by high-profile attorneys Bobby Samini and Mark Geragos, Zillow is accused of failing to pay wages, failing to pay overtime pay, and failing to provide meal and rest breaks. Zillow responded that "the narrative being pushed by this law firm through their multiple lawsuits is completely inconsistent with those who know and work with Zillow...the behavior described does not accurately depict our culture or the 1,200 Zillow employees."

In addition, Samini and Geragos represented a former Zillow employee in a sexual harassment action against the company, alleging "sexual torture" and “the most heinous acts of sexual harassment imaginable.” According to the lawsuit, Zillow's Southern California office represents an “adult frat house where sexual harassment and misconduct are normalized, condoned, and promoted by male managers.” Based on the allegations against the company, Samini has called Zillow a "modern day Animal House." On May 5, 2016, Zillow settled the action for an undisclosed amount, without admitting any wrongdoings.

In 2017, Zillow sent a cease and desist letter to the author of McMansion Hell, a blog of original content which satirized photos of ostentatious homes found on the site. Zillow's claims were described by legal experts as lacking merit and the company was accused of using its resources to infringe on free speech.