Walter Mischel was an influential psychologist best known for his work on personality and self-control. He gained fame for the Marshmallow Experiment, a study conducted in the 1960s that explored delayed gratification in children. In this experiment, children were given a choice between one marshmallow immediately or two marshmallows if they could wait for a short period. Mischel's research highlighted the importance of self-control and its impact on future success. He argued that personality is not fixed but can change depending on the situation. His work has had a lasting influence on psychology, particularly in understanding behavioral science and cognitive psychology.