Starting today, advertisers who use AdWords conversion tracking and receive more than 500 conversions from their cost-per-click (CPC) or cost-per-thousand impressions (CPM) campaigns in the most recent 30-day period will be invited to join this beta test, on a rolling basis. Eligible advertisers will see an alert in their AdWords account informing them that they can now try the PPA beta.As a refresher, pay-per-action advertising is a new pricing model that allows you to pay only for completed actions that you define (such as a lead, a sale, or a page view), after a visitor has clicked on your ad on a publisher's site. With CPC and CPM campaigns, advertisers need to continuously monitor and tune their campaigns to meet their target CPA (cost-per-action). With pay-per-action campaigns, advertisers only need to set their desired cost-per-action and pay for completed actions to hit their CPA targets.PPA ads will appear on publisher sites in the Google content network. Publishers are free to choose the PPA ads most relevant to their site and run them in new ad units.