WHOIS (pronounced "who is"; not an acronym) is a TCP-based query/response protocol which is widely used for querying an official database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. WHOIS lookups were traditionally made using a command line interface, but a number of simplified web-based tools now exist for looking up domain ownership details from different databases. Web-based WHOIS clients still rely on the WHOIS protocol to connect to a WHOIS server and do lookups, and command-line WHOIS clients are still quite widely used by system administrators.

The WHOIS system originated as a method that system administrators could use to look up information to contact other IP address or domain name administrators (almost like a "white pages"). The use of the data that is returned from query responses has evolved from those origins into a variety of uses including:

1. Supporting the security and stability of the Internet by providing contact points for network operators and administrators, including ISPs, and certified computer incident response teams;

2. Allowing users to determine the availability of domain names;

3. Assisting law enforcement authorities in investigations, in enforcing national and international laws, including, for example, countering terrorism-related criminal offenses and in supporting international cooperation procedures. In some countries, specialized non governmental entities may be involved in this work;

4. Assisting in the combating against abusive uses of ICT's, such as illegal and other acts motivated by racism, racial discrimination, xenophobia, and related intolerance, hatred, violence, all forms of child abuse, including paedophilia and child pornography, the trafficking in, and exploitation of, human beings.

5. Facilitating inquiries and subsequent steps to conduct trademark clearances and to help counter intellectual property infringement, misuse and theft in accordance with applicable national laws and international treaties;

6. Contributing to user confidence in the Internet as a reliable and efficient means of information and communication and as an important tool for promoting digital inclusion, e-commerce and other legitimate uses by helping users identify persons or entities responsible for content and services online; and

7. Assisting businesses, other organizations and users in combating fraud, complying with relevant laws and safeguarding the interests of the public.

Presently ICANN is undertaking a study to determine the uses and abuses of whois information. Other studies that are ongoing concern the accuracy of whois information, and the effectiveness of the processes for reporting inaccurate public whois information.

Due to the potential vulnerability of WHOIS information to improper manipulation, the legal owner of the domain is considered to be whoever controls the domain's username/passwords, e-mail address, and administrative features.