  The disks of spiral galaxies commonly show a lopsided mass distribution, with
a typical fractional amplitude of 10 % for the Fourier component m=1. This is
seen in both stars and gas, and the amplitude is higher by a factor of two for
galaxies in a group. The study of lopsidedness is a new topic, in contrast to
the extensively studied bars and two-armed spirals (m=2). Here, first a brief
overview of the observations of disk lopsidedness is given, followed by a
summary of the various mechanisms that have been proposed to explain its
physical origin. These include tidal interactions, gas accretion, and a global
instability. The pattern speed of lopsidedness in a real galaxy has not been
measured so far, the various issues involved will be discussed. Theoretical
studies have shown that the m=1 slow modes are long-lived, while the modes with
a moderate pattern speed as triggered in interactions, last for only about a
Gyr. Thus a measurement of the pattern speed of lopsided distribution will help
identify the mechanism for its origin.
