Oral habits can be classified in several ways, including by their nature (functional, muscular, etc.), cause (physiological, pathological), or degree of compulsiveness. Thumb sucking in particular progresses through three clinical phases: a normal initial phase, a significant phase where intervention may be needed, and a final intractable phase requiring psychological consultation. Theories on thumb sucking include psychosexual, learning/conditioning, and rooting reflex perspectives. Prolonged thumb sucking can result in dental malocclusions like an open bite from upper incisor proclination and lower incisor retroclination. Treatment is most effective during the significant second phase before deviations become permanent.