  Mass accretion is the key factor for evolution of galaxies. It can occur
through secular evolution, when gas in the outer parts is driven inwards by
dynamical instabilities, such as spirals or bars. This secular evolution
proceeds very slowly when spontaneous, and can be accelerated when triggered by
companions. Accretion can also occur directly through merging of small
companions, or more violent interaction and coalescence. We discuss the
relative importance of both processes, their time-scale and frequency along a
Hubble time. Signatures of both processes can be found in the Milky Way. It is
however likely that our Galaxy had already gathered the bulk of its mass about
8-10 Gyr ago, as is expected in hierarchical galaxy formation scenarios.
