The Lemaitre–Tolman class of cosmological models (spherically symmetric inhomogeneous metrics obeying the Einstein equations with a dust source) is derived and discussed in much detail, from the point of view of its geometry and its applications to cosmology. It is shown that these metrics can be used to describe the formation of cosmic voids and of galaxy clusters out of small perturbations of homogeneity at the time of emission of the cosmic microwave background radiation. Apparent horizons for central and noncentral observers, the formation of black holes, the existence and avoidance of shell crossings, the equations of redshift and the generation and meaning of blueshift are discussed. A simple example of a shell focussing singularity is derived. Among the cosmological applications are: solving the horizon problem without inflation, mimicking the accelerating expansion of the Universe by mass-density inhomogeneities in a decelerating model, drift of light rays, lagging cores of Big Bang, misleading conclusions drawn from observed mass distribution in redshift space.

The family of the Szekeres–Szafron, shell solutions of Einstein’s equations, is derived and discussed in detail. The discussion contains, among other things, the interpretation of the Szekeres coordinates, the invariant definitions of the whole family, the class II ($\beta,_z = 0$) family as a limit of the class I family, matching the Szekeres metric to the Schwarzschild metric (the class II S metric can be matched to Schwarzschild only inside the event horizon), conditions for absence of shell crossings, the description of the mass dipole, the apparent horizons, the Goode–Wainwright representation and a brief listing of recommended further reading on geometric and astrophysical properties of these solutions.