Thorndike is perhaps best-known for the theory he called the law of effect, which emerged from his research on how cats learn to escape from puzzle boxes.

Thorndike's theory of learning, also known as the law of effect, states that behavior that is followed by a positive consequence is more likely to be repeated, while behavior that is followed by a negative consequence is less likely to be repeated.

Thorndike's theory was an association theory, as many were in that time. He believed that the association between stimulus and response was solidified by a reward or confirmation. He also thought that motivation was an important factor in learning.

Thorndike graduated from Wesleyan University in 1895. He studied animal behaviour with William James at Harvard University (1895–97) and with James McKeen Cattell at Columbia University, where he received his Ph. D. (1898) and where he spent most of his career.