The current study sought to examine the impact of strengthening cigarette pack warnings on attention, message processing, and perceived effectiveness, through a systematic review of longitudinal observational studies. The review included 22 studies (N = 81,824 participants). Strengthened warnings increased attention to warnings, recall of warnings, and thinking about the health risks of smoking. Strengthened warnings also increased several perceived effectiveness outcomes, including perceptions that warnings reduce smoking and motivate quitting. Strengthened cigarette pack warnings achieve their goal of attracting attention and enhancing motivation to act. Strengthening warning policies should be a priority for tobacco control globally.