Companies that specialize in advertising, acting as a resource to brands to help plan, buy, and measure advertising campaigns across marketing channels. They are uniquely positioned to help provide broader, cross-industry expertise, advising marketers on how to best represent their brand and most efficiently allocate marketing dollars to reach consumers. Within an agency, there might be an Agency Trading Desk (ADT), an independent unit that buys on their behalf, with the idea that by bringing things in-house through the use of technology, you can operate more efficiently and transparently.