For the use of the term testimonial in sport (especially football) see testimonial match.

In promotion and of advertising, a testimonial or endorsement consists of a written or spoken statement, sometimes from a public figure, sometimes from a private citizen, extolling the virtue of some product. The term "testimonial" most commonly applies to the sales-pitches attributed to ordinary citizens, whereas "endorsement" usually applies to pitches by celebrities. See also Testify, Testimony, for historical context and etymology.

Written testimonials in the history of advertising

Testimonials are in the form of letters and ad copy featured very commonly in the advertising of patent medicines in the 19th and 20th centuries. The pages of almanacs and other promotional literature filled up with multiple testimonials,some with accompanying photographs, that tell of the healing powers of the products in question. Dr. R. V. Pierce, marketer of Pierce's Golden Medical Discovery, published The People's Common Sense Medical Adviser in 1875; its publication continued for forty years. In addition to a fair explanation of medical knowledge at the time it appeared, this book contained literally hundreds of testimonials extolling Pierce's nostrums, or talking up the virtues of Pierce's Buffalo, New York clinic. Stern photographs of women who owe the cure of their "female weakness" to Pierce's medicines accompany many of the letters. A Denver, Colorado man's testimonial affirms that they put an end to his self-abuse:

Case 461,306. Onanism. Melancholia; contemplated suicide.

Gentlemen — Having waited several weeks after finishing the last medicine, to see if there would be any relapse, I now send you a report of treatment. I believe I am thoroughly cured, not only of poor health, but of all desire to abuse myself. I have regained health, spirits, and confidence. Am married, something I have long desired, but never before dared to attempt. Please accept my sincere thanks, gentlemen. Your medicine has saved me from a suicide's grave.

--- H., Denver, Col.

Not only anonymous persons, but occasionally politicians, entertainers, and other celebrities offered their endorsements to the vendors of patent medicine. The makers of Vin Mariani, a cocaine-laced patent medicine, secured one of the most valuable testimonials ever by receiving the recommendation of Pope Leo XIII. Queen Victoria also endorsed a number of patent medicines and other products, and the frequently-seen notices touting a manufacturer or a product "by appointment to" a monarch or his family continue the practice of royal endorsement in a somewhat more low-key manner.

Such coups came towards the end of the era of written testimonials. Later advertisers found that no one bothered to read the testimonials anymore; the sheer bulk of their numbers made them no more convincing or appealing. A warier public wondered whether these anecdotes really proved anything, and often doubted their genuineness.

Health products remain one of the more prominent marketing segments in which testimonials retain some effectiveness. Due to the placebo effect and to people's reluctance to expose their frailties to apparently remote and opaque medical doctors, cures for frailties both physical and mental, both real and imagined, continue to sell. A popular generic name for such quack nostrums has come about: "snake oil".

In response to the "snake oil salesman" referred to above and their fake testimonials, a new arena of web site 'safety' or 'ethics enforcement' has arisen in the form of third party testimonial verification Companies such as Truth Whale.com independently verify the authenticity of businesses proposed customer testimonials. Once the testimonials are verified authentic the company may include a 'web stamp' or 'web seal' indicating that the testimonials displayed have been third party verified. Such proof of authenticity regarding previous customer testimonials has been proven to dramatically increase the sales of products and services as it's now akin to celebrity endorsement in the fact that the testimonials are at least real, and no longer just some stranger (ie. "Joe Brown, NY, NY"). Although still a stranger, a potential customer of said company now at least knows the testimonials are indeed true because they have been third party verified.

Measuring the use of celebrities in marketing programs

Advertisers have attempted to quantify and qualify the use of celebrities in their marketing campaigns by evaluating their awareness, appeal, and relevance to a brand's image and the celebrity's influence on consumer buying behavior.

For example, Omnicom agency Davie Brown Entertainment has created an independent index for brand marketers and advertising agencies that determines a celebrity’s ability to influence brand affinity and consumer purchase intent. According to the Wall Street Journal, the so-called "Davie-Brown Index" will "enable advertisers and ad-agency personnel to determine if a particular public figure will motivate consumers who see them in an ad to purchase the product advertised."

Celebrity endorsements have proven very successful in China where, due to increasing consumerism, it is considered a status symbol to purchase an endorsed product. On August 1, 2007 laws were passed banning healthcare professionals and public figures such as movie stars or pop singers from appearing in advertisements for drugs or nutritional supplements. A spokesperson stated: "A celebrity appearing in drug advertising is more likely to mislead consumers, therefore, the state must consider controlling medical advertisements and strengthen the management of national celebrities appearing in medical advertisements." China had already banned its own athletes from taking part in any advertising and public relations work in 2006.