In tunnel fires, the most immediate threat to life is not the direct exposure to fire, but smoke inhalation. Efficient control of smoke propagation, therefore, is one of the most important issues in designing tunnel ventilation and a full understanding of the characteristics of smoke propagation in tunnels is a necessity in order to proceed with a successful design. In the present article, we review the progress of research on smoke propagation in tunnels, wherein the tests in full-scale tunnels, the nature of fire, the computational fluid dynamic-field model approach (CFD-FMA), and the Froude number preservation approach (FNPA) are discussed. The gravity current approach (GCA) is also developed to predict the smoke propagation behavior in tunnels and a CFD-FMA example is given from which the features of smoke propagation can be closely examined. The analytical results from FNPA indicate that, in the upstream of fire, the critical ventilation velocity is generally proportional to the one-third power of the heat release rate (HRR); some modifications to this power law are necessary for special cases. In the downstream of fire, the GCA results show that smoke propagates along the tunnel with a constant speed, which is essentially linearly proportional to the ventilation velocity. The numerical results from CFD-FMA determine a safety domain in terms of the ventilation velocity and the HRR of fire. In view of rapidly increasing computational power, the CFD-FMA is becoming a major approach in studying smoke propagation in tunnels, while the GCA and FNPA are useful in engineering design. This review article includes 60 references.