Nuclear star clusters, that fragment into metal-poor stars at the centers of forming galaxies, provide ideal environments for the formation of massive black holes. Stellar collisions and black hole ejections contribute to the dynamical formation of intermediate-mass black holes via repeated mergers of massive stellar black-hole seeds. Including gas accretion and galaxy mergers allows supermassive black holes to form at early epochs. There are potential gravitational-wave signatures of the supermassive black-hole formation process.