Spamming is the use of messaging systems to send multiple unsolicited messages (spam) to large numbers of recipients for the purpose of commercial advertising, non-commercial proselytizing, or any prohibited purpose (especially phishing). It can also be repeatedly sending the same message to the same user. The most widely recognized form of spam is email spam.

Similar abuses also occur in other media: instant messaging, Usenet newsgroup, Web search engines, blogs, wiki, online classified ads, mobile phone messaging, Internet forum, fax transmissions, social spam, mobile apps, television advertising, and file sharing. It is named after Spam, a luncheon meat, by way of a Monty Python sketch about a restaurant that has Spam in almost every dish in which Vikings annoyingly sing "Spam" repeatedly.

Spamming remains economically viable because advertisers have no operating costs beyond the management of their mailing lists, servers, infrastructures, IP ranges, and domain names, and it is difficult to hold senders accountable for their mass mailings. The costs, such as lost productivity and fraud, are borne by the public and by Internet service providers, which have added extra capacity to cope with the volume. Spamming has been the subject of legislation in many jurisdictions.

A person who creates spam is called a spammer.