A key tenet of safety culture is the premise that all team members should speak up about safety concerns; however, prior research has found that many healthcare workers do not feel comfortable speaking up. This systematic review explored the mechanisms influencing speaking up behaviors while witnessing medical errors. Several studies identified hierarchies as a barrier to speaking up, particularly among trainees, due to fear of retribution, being exposed, or undermining senior clinicians. The findings from this review reinforce the concept that communication is a learned skill that requires ongoing training. The authors suggest that future studies on speaking up behavior examine the causal mechanisms influencing speaking up behavior.