  We present evolutionary models for young low mass stars and brown dwarfs
taking into account episodic phases of accretion at early stages of the
evolution, a scenario supported by recent large surveys of embedded protostars.
An evolution including short episodes of vigorous accretion ($\mdot \ge 10^{-4}
\msolyr$) followed by longer quiescent phases ($\mdot < 10^{-6} \msolyr$) can
explain the observed luminosity spread in HR diagrams of star forming regions
at ages of a few Myr, for objects ranging from a few Jupiter masses to a few
tenths of a solar mass. The gravitational contraction of these accreting
objects strongly departs from the standard Hayashi track at constant $\te$. The
best agreement with the observed luminosity scatter is obtained if most of the
accretion shock energy is radiated away. The obtained luminosity spread at 1
Myr in the HR diagram is equivalent to what can be misinterpreted as a $\sim$
10 Myr age spread for non-accreting objects. We also predict a significant
spread in radius at a given $\te$, as suggested by recent observations. These
calculations bear important consequences on our understanding of star formation
and early stages of evolution and on the determination of the IMF for young
($\le$ a few Myr) clusters. Our results also show that the concept of a stellar
birthline for low-mass objects has no valid support.
