New technologies like large-scale social media sites (e.g., Facebook and Twitter) and crowdsourcing services (e.g., Amazon Mechanical Turk, Crowdflower, Clickworker) are impacting social science research and providing many new and interesting avenues for research. The use of these new technologies for research has not been without challenges, and a recently published psychological study on Facebook has led to a widespread discussion of the ethics of conducting large-scale experiments online. Surprisingly little has been said about the ethics of conducting research using commercial crowdsourcing marketplaces. In this article, I focus on the question of which ethical questions are raised by data collection with crowdsourcing tools. I briefly draw on the implications of Internet research more generally, and then focus on the specific challenges that research with crowdsourcing tools faces. I identify fair pay and the related issue of respect for autonomy, as well as problems with the power dynamic between researcher and participant, which has implications for withdrawal without prejudice, as the major ethical challenges of crowdsourced data. Furthermore, I wish to draw attention to how we can develop a “best practice” for researchers using crowdsourcing tools.