  We investigate Higgs boson pair production at hadron colliders for Higgs
boson masses m_H\leq 140 GeV and rare decay of one of the two Higgs bosons.
While in the Standard Model the number of events is quite low at the LHC, a
first, albeit not very precise, measurement of the Higgs self-coupling is
possible in the gg -> HH -> b\bar{b}\gamma\gamma channel. A luminosity-upgraded
LHC could improve this measurement considerably. A 200 TeV VLHC could make a
measurement of the Higgs self-coupling competitive with a next-generation
linear collider. In the MSSM we find a significant region with observable Higgs
pair production in the small \tan\beta regime, where resonant production of two
light Higgs bosons might be the only hint at the LHC of an MSSM Higgs sector.
