Despite its popularity, search engine advertising is a particularly complex and demanding technique. One of the main challenges for Google Ads managers is to adequately monitor performance. Indeed, the literature identifies a plethora of metrics to measure the success of a search engine ads campaign. One research question arises: What are the metrics adopted by small and medium-sized companies to measure the performance of a Google Ads campaign? This chapter includes a mixed-method study with digital marketing professionals experienced in managing Google Ads campaigns for Portuguese SMEs. Interviews helped highlight the main difficulties faced by SEM's Google Ads' managers and to identify the performance measures they mostly control. Then, a survey enabled to analyse the association between performance measures and campaigns' perceived success. The insights produced by this chapter are particularly interesting for researchers, teachers, business managers, and digital marketing professionals, as it presents important clues on measuring the effectiveness of Google Ads campaigns.