  Black holes (BHs) with masses between 100 to 100,000 times the mass of the
Sun ($\rm{M}_{\odot}$) are classified as intermediate-mass black holes (IMBHs),
potentially representing a crucial link between stellar-mass and supermassive
BHs. Stellar-mass BHs are endpoints of the evolution of stars initially more
massive than roughly 20 $\rm{M}_{\odot}$ and generally weigh about 10 to 100
$\rm{M}_{\odot}$. Supermassive BHs are found in the centre of many galaxies and
weigh between $10^{6}$ to $10^{10} \ \rm{M}_{\odot}$. The origin of
supermassive BHs remains an unresolved problem in astrophysics, with many
viable pathways suggesting that they undergo an intermediate-mass phase.
Whether IMBHs really stand as an independent category of BHs or rather they
represent the heaviest stellar mass and the lightest supermassive BHs is still
unclear, mostly owing to the lack of an observational smoking gun. The first
part of this chapter discusses proposed formation channels of IMBHs and focuses
on their formation and growth in dense stellar environments like globular and
nuclear star clusters. It also highlights how the growth of IMBHs through
mergers with other BHs is important from the point of view of gravitational
waves and seeding of supermassive BHs in our Universe. The second part of the
chapter focuses on the multi-wavelength observational constraints on IMBHs in
dense star clusters and dwarf galactic nuclei. It also examines the potential
insights that future gravitational wave detectors could offer in unraveling the
mystery surrounding IMBHs.
