There are four very influential inventions that have shaped the media and thus the advertising industry - the printing press, radio, television and the Internet. The printing press made the wide dissemination of information with words on paper possible, mainly advertisements in newspapers and magazines. Selling material had to be created and advertising agencies were born.

The first advertising agency, Volney B. Palmer, was opened in Philadelphia in 1841. By 1861 there were 20 advertising agencies in New York City alone. Among them was J. Walter Thompson, today the oldest American advertising agency in continuous existence. Radio became a commercial medium in the 1920s.

For the first time, advertising could be heard, not just seen. Soap operas, music, and serial adventures populated the new medium, and as radios appeared in virtually every home in America, sales of products advertised on the air soared. Advertisers rushed to write infectious advertising jingles, an art form that still has its place in the advertising repertoire of today.

Then television changed everything. Although TV was invented in the 1920s, it didn't become a mass commercial medium until the 1950s when the prices of television sets began to approach affordability. Print and radio had to take a back seat because, for the first time, commercials were broadcast with sight, sound and motion.

The effect of the telvesion on the advertising industry and the way products were sold was remarkable. Advertising agencies not only had to learn how to produce these mini movies in units of 30 and 60 seconds, they had to learn to effectively segment the audience and deliver the right commercial message to the right group of consumers.

Cable television was the next great innovation, offering a greater variety of channels with more specific program offerings. That allowed advertisers to narrowcast. Before the advent of cable television, the networks attempted to reach demographics by airing at different times throughout the broadcast period. Soap operas were broadcast during the day to reach women, news in the evening to reach an older target audience.

Cable television, on the other hand, brought with it channels like MTV that catered to young music lovers, ESPN, for (typically) male sports fans, and the Food Network, for people who love cooking (or at least love to watch others cook). These new advertising channels were delightful for advertisers who wished to target certain audiences with specific interests, though less so for the networks who saw their share of ad revenue dwindle.