Nowadays, many companies contract their testing functions out to third-party IT outsourcing companies. This process, referred to as test outsourcing, is common in the industry, yet it is rarely studied in the research community. To bridge this gap, we performed an empirical study on test outsourcing with 10 interviewees and 140 survey respondents. We investigated various research questions and found that customer satisfaction expectations, tight project schedule, and domain unfamiliarity are the top three challenges testers face.