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The physics of indirectly driven targets for inertial confinement fusion — so-called hohlraum targets — is investigated. Scaling relations for radiation heat waves in high-Z and low-Z materials are derived from one-dimensional multigroup simulation. A two-temperature model is developed for radiation cavities including fusion capsules. The efficiency of X-ray transfer to the capsule by multiple absorption and re-emission inside the cavity is obtained as a function of cavity areas and materials. Using gold for the cavity wall and carbon for the capsule ablator, transfer efficiencies between 50% and 33% are obtained for area ratios between 5 and 10, respectively. Also the hydrodynamic efficiency of X-ray driven capsule implosion and the dependence of the implosion velocity on the hohlraum temperature are given analytically, derived from the rocket model. With carbon ablators, hydroefficiencies of up to 20% can be achieved. Under optimal conditions, an implosion velocity of 3 × 107 cm/s is reached with a temperature of about 210 eV of the capsule ablator and about 250 eV of the cavity wall. Assuming 70–90% conversion efficiency of beam energy into X-rays (not analysed in this paper), overall coupling efficiencies in the range of 5–10% seem to be possible. One-dimensional simulations of full reactor size targets (10 MJ driver pulses) are presented. The model results compare well with the simulations. Limits in scaling down to smaller systems are discussed; the scaling relation for the required enhancement of implosion velocity and hohlraum temperature is derived.