Anomalies arise, Kuhn argues, when normal science encounters results that cannot be forced into the existing paradigm. It is not uncommon, he acknowledges, for the results of experiments to differ from what is expected. Normally, what scientists then attempt to do is to correct for their errors, find a way to assimilate the results into the existing paradigm, or simply drop the results as anomalous. However, when anomalies become undeniable, persuasive, or intriguing, they can give way to crises, which then produce periods of scientific uncertainty from which a revolution can emerge.

According to Kuhn, normal science, the phase during which scientists work within an accepted paradigm, is marked by routine puzzle-solving activities. Scientists focus on refining existing theories and conducting experiments that fit within the established framework. However, over time, anomalies accumulate—observations or results that defy easy explanation within the existing paradigm. These anomalies create a growing tension and dissatisfaction within the scientific community as they challenge the paradigm's ability to account for all observed phenomena.

Despite the critical role anomalies play in scientific revolutions, Kuhn notes that scientists, particularly during periods of normal science, often exhibit a resistance to anomalies.