The evaluation of search engine marketing companies is based on a few important factors. Among the most significant are the following: the number of years in business; the combined industry experience; the number of employees; the size of the clients; the client and employee turnover; the depth and breadth of the service offering; the pricing; the proprietary services; the credibility of the industry; the success stories and testimonials of the clients; and the business's philosophy and process. As "search engine marketing" is a highly competitive phrase, consider how visible their clients are in comparison to their own. Those that do well often have a bit too much time and not enough client work to keep them occupied. If they don't have endorsements and success stories, be sure to ask for references.