This talk will be about the paper with the title stated above that Demetrios Christodoulou published in 1991. In this work, Christodoulou studies solutions to the Einstein Vacuum Equations in spherical symmetry und proved that under certain assumptions on the mass and radii of spheres in an annular region of the initial future cone, a trapped sphere will form in the future. After Penrose's theorem, a trapped sphere is an indicator for geodesic incompleteness or closed timelike curves. With the spherical symmetry and the curvature assumptions, we will derive a metric and a system of equations containing information about mass, radii and curvature and use this in order to derive the desired result with some estimates from monotonicity arguments.