The Madden-Julian oscillation (MJO) is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The eff ect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher-frequency eddies are delineated.