Some key physical aspects of the inertial confinement fusion (ICF) are discussed. The minimum scale of ICF microexplosions is determined by the ability to implode spherical shells with high radial convergence ratios CR and high initial aspect ratios AR0. The attainable values of CR are limited by large-scale drive asymmetries, while the values of AR0 are constrained by the Rayleigh-Taylor instability. Under the indirect drive approach to ICF, it is easier to achieve the required uniformity of the drive pressure, but the penalty is a factor 4-5 reduction of the target energy gain as compared to the direct drive option. Spark ignition is a crucial issue for indirect drive targets (at least for those to be used in power reactors), while the targets driven directly by heavy ion beams could, in principle, utilize a less demanding volume ignition scheme.