The light distribution in many spiral galaxies is observed to be lopsided, with the spatial extent much larger along one side, as in M 101. This indicates an azimuthal mass asymmetry (m=1). Lopsidedness is ubiquitous and occurs in stars and gas, in the outer disk as well as the central region. Its typical measured amplitude is high (~ 10 %), making it a common feature of spiral galaxies. The physical mechanisms proposed for its origin include tidal interactions and gas accretion. The lopsidedness has a strong impact on the dynamics and evolution of a galaxy.The central regions of merger remnants also exhibit strong and long-lived lopsidedness. In this talk, I will briefly summarize the observational and theoretical aspects of this exciting topic and the related open problems.