Luminosity challenges for next generation e+e- and pp colliders

Collider facilities now being designed demand luminosities about 100 times those now in existence. Such luminosity goals present significant technical challenges. These challenges are discussed for ring on ring, linac on ring and purely linear colliders. Goals for ring on ring colliders can be evaluated using extensive current knowledge and appear achievable if difficult. Evaluation of linac on ring and linear colliders is more uncertain owing to the relatively early state of these devices and the considerable distance in their proposed operating regimes from today's practice