This chapter is part of the free e-book 'Basics of Search Engine Marketing', published in May 2005. As mentioned in chapter 3, pay per click advertising is the main source of revenue for today’s search engines. The details that I will describe here will not be entirely accurate in the future, as the PPC environment changes rapidly. Regardless, the basic concepts will remain the same in the near future. The PPC providers can be grouped into three categories: tier 1, tier 2 and tier 3, based on the size and popularity of the provider.