Over the past few decades, Tully's fewest switches surface hopping (FSSH) has been widely utilized due to its ease of implementation and acceptable balance between efficiency and reliability. However, it is well known that FSSH suffers from the overcoherence problem, which leads to inconsistent populations derived from wave function and active states. To solve this problem, various methods with decoherence effects have been proposed. Despite of these great successes, a rigorous theoretical foundation of the SH framework is still needed to realize systematic improvements.