Online advertising or web advertising is the central and most important promotional activity of modern brands and companies on the global network – the World Wide Web, which encourages the purchase of goods and services, either over the Internet, either at the physical point of sale. Online advertising includes banners, the pages displayed during the web search, contextual ads, blogs, advertising on social networks, interstitial ads, Email ads... . In the most general sense, online advertising is defined as advertising (commercial) messages that appear on a computer monitor, using an online service called Internet World Wide Web.  The very fact that we say “central” promotional activity, suggests that there are other Internet promotional activities such as Internet PR, Internet direct marketing and so on. Thus, the concepts of online promotion and online marketing are more general than the concept of online advertising. Their relation, in the terms of generality, is the same as the relation between the concepts of marketing, promotion and advertising in traditional media. Consequently, online advertising is part of the overall marketing and promotional activities of the participants at the national or international market. In everyday use of terminology, the concepts of online marketing and online advertising are often equated, which also tells a lot about the level of significance of online advertising in the overall system of promotion and internet marketing. On the other hand, online advertising has its own internal structure, because till now there has been a differentiation of several forms of this type of advertising. Inside this structure we can distinguish: display advertising, advertising on web search engines, classified ads and advertising in the address books / directories / catalogs, and more. These forms of internet advertising are dominant in its structure. Display advertising is popularly called banner advertising, but it is certainly a broader phenomenon, because the banners have become in recent years only one of the possible forms of display advertising. Those other forms are video ads, branding, ads in the text and others. Web search engines such as Google, Yahoo, MSN, Bing are significant platform for advertising on the Internet, given that they are practically unavoidable mediums in the Internet use. For this reason, many brands and companies do their advertising on the most important search engines; in other words, they pay for a good position of online contents about them, in the search of the Internet users. Small ads on the Internet and advertising in address books and other types of directories have their own stable and indispensable place in the overall web advertising. These types of online advertising also record revenue growth, but it must be noted that display and search advertising record a higher level of growth and at the same time, as the segments, strengthen dominance. Online advertising is of predominant importance compared to all the other forms of promotional activities on the web, if we take into account the amount of money invested by the world’s largest multinational brands in display and search advertising, on one side, and in PR announcements and press releases on various influential news sites, on the other side. Another fact which largely indicates the importance of online advertising is its very strong expansion in relation to the advertising on the so-called traditional media such as TV, radio, newspapers, billboards and so on., at least when we observe the group of the most developed countries in the world (EU, USA, Canada, Oceania, Japan, etc.). Most predictions about the commercial position of the Internet media in the future suggest its predominance over the traditional media. However, there are other views, such as the one represented by the marketing guru, Philip Kotler. He believes that the Internet will not become the main medium of advertising in the future, such as TV, radio and newspapers, because commercials (advertising) are not welcome by Internet users  In some countries, online advertising, according to the total revenue, has been equated with advertising on TV, which is the most powerful traditional media. In the UK, in 2009 online advertising surpassed TV advertising, when the share of the first one was 23.5% and the second one 21.9%. The cause of this shift is not only the online advertising growth of 4.6 percent compared to 2008, but also the decline in television advertising in the amount of 17%. The institutions of online advertising can be defined as a network of institutions and organizations that are crucial to the mere process of conducting the business of online advertising. Institutional base is essential for the formation, sustainability and development of any business, because the mere institutions and organizations are the entities that conduct and improve business. The network of online advertising in Serbia is, in structural sense, in the process of rapid development, which means that the indispensable institutions and organizations for online advertising have been formed. On the other hand, the systematic relation between these institutions is weak and their importance and role in the field of online advertising are not at sufficient level; they must be improved as soon as possible, so as to avoid the delay of Serbia, not only in relation to Europe and the world, but also in relation to the countries of the region.