Okubo-Zweig-Iizuka Rule Violation and Possible Explanations

Violations of the Okubo-Zweig-Iizuka (OZI) rule in $s\bar s <-> n\bar n = 1/2 (u\bar u + d\bar d)$ mixing and $\phi$ production from nucleon-antinucleon annihilation are reviewed. Possible explanations are examined. We conclude that the two-step hadronic loops can explain these OZI violations naturally with proper consideration of cancellations between loops. No conclusive evidence exists for glueball states, $s\bar sn\bar n$ four quark states, instanton effects, and strange quarks in nucleon.