The Stroop effect, a classic phenomenon in cognitive psychology, has been studied for over 50 years, with more than 400 studies. This review identifies 18 reliable empirical findings that must be explained by any successful theory of the Stroop effect. Existing theories, such as the relative speed of processing and automaticity of reading, are found to be insufficient. Recent theories emphasizing parallel processing of the irrelevant and relevant dimensions are more promising. The Stroop task, first described by J. R. Stroop in 1935, remains influential due to its ability to tap into fundamental cognitive processes and its robustness. The task involves naming the color of inked words, which often conflicts with the word itself, creating interference. Stroop's work built on earlier research by James McKeen Cattell, who noted that naming colors was slower than reading words due to the automaticity of reading. Stroop's experiments demonstrated that interference occurs when the ink color of a word conflicts with the word itself. In Experiment 1, reading words in incongruent colors did not show significant interference, while in Experiment 2, naming colors of incongruent words showed significant interference. Stroop also found that interference decreased with practice, indicating a learning effect. The Stroop effect has been extensively studied, with numerous variations and analogs developed over the years. These include the picture-word interference task, auditory analogs, and other tasks involving conflicting stimuli. The effect is often attributed to response competition and the automaticity of reading. However, recent research suggests that parallel processing of the relevant and irrelevant dimensions may be more accurate. The review highlights the importance of empirical findings in understanding the Stroop effect, emphasizing the need for theories that can account for the complexity of the phenomenon. The Stroop effect remains a critical area of research, with ongoing studies exploring its implications for cognitive processes and individual differences.The Stroop effect, a classic phenomenon in cognitive psychology, has been studied for over 50 years, with more than 400 studies. This review identifies 18 reliable empirical findings that must be explained by any successful theory of the Stroop effect. Existing theories, such as the relative speed of processing and automaticity of reading, are found to be insufficient. Recent theories emphasizing parallel processing of the irrelevant and relevant dimensions are more promising. The Stroop task, first described by J. R. Stroop in 1935, remains influential due to its ability to tap into fundamental cognitive processes and its robustness. The task involves naming the color of inked words, which often conflicts with the word itself, creating interference. Stroop's work built on earlier research by James McKeen Cattell, who noted that naming colors was slower than reading words due to the automaticity of reading. Stroop's experiments demonstrated that interference occurs when the ink color of a word conflicts with the word itself. In Experiment 1, reading words in incongruent colors did not show significant interference, while in Experiment 2, naming colors of incongruent words showed significant interference. Stroop also found that interference decreased with practice, indicating a learning effect. The Stroop effect has been extensively studied, with numerous variations and analogs developed over the years. These include the picture-word interference task, auditory analogs, and other tasks involving conflicting stimuli. The effect is often attributed to response competition and the automaticity of reading. However, recent research suggests that parallel processing of the relevant and irrelevant dimensions may be more accurate. The review highlights the importance of empirical findings in understanding the Stroop effect, emphasizing the need for theories that can account for the complexity of the phenomenon. The Stroop effect remains a critical area of research, with ongoing studies exploring its implications for cognitive processes and individual differences.