  The currently discussed theories of bulge formation are reviewed, including
the primordial scenario, where bulges form rapidly and then accrete disks, the
secular scenario, where bulges are formed by dynamical evolution of disks
through bars and galaxy interactions, and some combinations of both, where
formation of bulges and disks are more continuous and interleaved. The various
scenarios make specific predictions about the relative masses, angular momenta,
colours, metallicities of bulges relative to disks, and the bulge-to-disk ratio
as a function of time. Dynamical processes relevant to the formation of bulges
(bar instabilities, mergers) are described and tested against observed
statistics. Current data suggest a dynamical feedback from gravitational
instabilities in bulge and disk formation. It is very difficult to discriminate
between the various scenarios from surveys at z=0 only, and observations at
high redshift are presently the best hope for large progress.
