Fact-checking has gained importance in recent years, as so-called “fake news” has started to spread on social media. News outlets and independent organizations engage in debunking to combat the massive spread of disinformation. However, several authors have argued that fact checkers can only be successful if they win the trust of the audience - by making their practices transparent. This article analyzes the degree of source transparency provided by eight fact checkers from different countries (the US, the UK, Germany, and Austria). The findings show major differences among the outlets studied which can be attributed to varying levels of journalistic professionalism as well as to organizational differences. Implications for the success of fact-checking and solutions to combat online disinformation are discussed.