  We review the present knowledge of disk accretion in young low mass stars,
and in particular, the mass accretion rate and its evolution with time. The
methods used to obtain mass accretion rates from ultraviolet excesses and
emission lines are described, and the current best estimates of mass accretion
rate for Classical T Tauri stars and for objects still surrounded by infalling
envelopes are given. We argue that the low mass accretion rates of the latter
objects require episodes of high mass accretion rate to build the bulk of the
star. Similarity solutions for viscous disk evolution suggest that the inner
disk mass accretion rates can be self-consistently understood in terms of the
disk mass and size if the viscosity parameter alpha is approximately 1e-2.
Close companion stars may accelerate the disk accretion process, resulting in
accretion onto the central star in less or equal than 1Myr; this may help
explain the number of very young stars which are not currently surrounded by
accretion disks (the weak emission T Tauri stars).
