Tolerance is commonly defined as putting up with ideas, persons, or practices that one disagrees with or dislikes. Theory and research have focused on one particular kind of tolerance, namely political tolerance, that concerns the willingness to extend civil liberties to disliked groups. Although difficult and complex, political tolerance has been considered a key to the sustenance of democratic and increasingly diverse societies. Knowledge of tolerance development and factors that influence this process is essential for fostering tolerance in societies. This entry summarizes key points in understanding tolerance and its development employing a scientific lifespan perspective.