Advertising Agency is an independent firm specializing in performance of marketing functions: conducting advertising research, develop plans for advertising, designing and implementation of advertising campaigns, including the preparation of promotional messages, production of advertising media, placement of advertising messages, etc. Advertising Agency makes about mentioned functions on behalf of their clients (costumers) - advertisers, who are responsible to cover accured expenses. The agency  has  financial, legal and ethical obligations to advertisers. The world's largest advertising agencies have turned into giant multinational corporations with complex organizational structure. Ten of the largest advertising companies in the world are given in Table.

From the data in Table, you can see that seven of the ten world's largest advertising companies are managed from the U.S. headquarters. This is directly related to the fact that the American advertising market is the most capacious in the world. Thus, the total advertising expenditures in this country in 2010 were estimated to be 42% of the whol advertising industry of top 10 advertising agencies.

Even the largest firms, advertisers, each with its own powerful advertising service, can not be without prejudice to the effectiveness of their business without the help of advertising agencies. This is due to the following reasons:

Table of largest advertising companies in the world  

Rating	Advertising Agency	Headquarters	Revenue (USD, Millions)
1	Omnicom Group	New York	10481,1
2	WPP Group	London	10032,2
3	Interpublic Group of Companies	New York City	6000,7
4	Publicis Group	Paris	5107,2
5	dentsu	Tokyo	2887,8
6	Havas	Surenes, France	1808
7	Aegis Group	London	1577,6
8	Hakuhodo Dy Holdings	Tokyo	1364
9	Asatsu-DK	Tokyo	444,8
10	MDC Partners	Toronto/New York City	443,5
		Total:	40146,9

Every Advertising agency daily with a wide range of marketing situations, allows him to gain a deeper understanding of the interests of the audience, skills and competence. In turn, this defines into a higher quality of advertising and the appropriate level of its effectiveness.
The Agency is an independent organization that allows advertisers to look at the problem "from outside", i.e., more objectively. Thus, decreasing the negative impact of such subjective factors as excessive dependence on advertising from the tastes of individual managers, incorrect installation on the expected response of consumers, etc.
Advertising agency, as a rule, has established relationships with the media, consistently and in advance of purchasing their time and place to advertise. Cooperation with advertising agency allows advertisers to quickly solve their problems, helps him save money and time.
Appeal to advertising agencies (especially those of them who provide a full range of advertising services) provides a systematic approach to advertising, which increases its effectiveness.