This literature review summarizes previous research on referee favoritism in soccer leagues. Several studies found evidence that referees display home team favoritism by awarding more penalties and yellow/red cards to away teams (Boyko et al., 2007; Buraimo et al., 2010). Referees also tended to add more injury time when the home team was losing by one goal compared to winning or drawing (Sutter and Kocher, 2004; Garciano et al., 2005; Scoppa, 2008). However, two more recent studies found only marginal evidence of these biases in the Premier League and Bundesliga (Neil and Witt, 2008; Riedl et al., 2015). The literature suggests crowd effects can