  We study the kinetic regime of the Bose-condensation of scalar particles with
weak $\lambda \phi^4$ self-interaction. The Boltzmann equation is solved
numerically. We consider two kinetic stages. At the first stage the condensate
is still absent but there is a nonzero inflow of particles towards ${\bf p} =
{\bf 0}$ and the distribution function at ${\bf p} ={\bf 0}$ grows from finite
values to infinity in a finite time. We observe a profound similarity between
Bose-condensation and Kolmogorov turbulence. At the second stage there are two
components, the condensate and particles, reaching their equilibrium values. We
show that the evolution in both stages proceeds in a self-similar way and find
the time needed for condensation. We do not consider a phase transition from
the first stage to the second. Condensation of self-interacting bosons is
compared to the condensation driven by interaction with a cold gas of fermions;
the latter turns out to be self-similar too. Exploiting the self-similarity we
obtain a number of analytical results in all cases.
