We analyse the dynamics of the collision of two spherical massive shel
ls in a generally spherically symmetric background, obtaining an expre
ssion from the conservation law that imposes a constraint between the
different parameters involved. We study the light-like limit and make
some comparisons of the predictions of our master equation with the re
sults obtained in the case of collision of light-like shells, like the
short life of white holes or the mass-inflation phenomena. We present
some particular cases of the constraint equation.