It is well-known that two given spacetimes with boundary can be matched if the so-called junction conditions are satisfied. In this talk, we will exploit the so-called formalism of hypersurface data (a robust formalism to study hypersurfaces from a completely abstract viewpoint) to provide a suitable abstract (in the sense of detached from the spacetimes to be matched) formulation of the junction conditions. Then, we will focus on the case when both boundaries are null. We will obtain explicitly the gravitational/matter content of a null shell as well as the energy-momentum tensor of the shell. The purely gravitational part of the shell is also derived. Generically, there exists at most one way of matching two spacetimes verifying the junction conditions. However, sometimes this is not so. We will also address the problem of multiple matchings, and provide the relationship between the matter contents corresponding to two different matchings of the same two spacetimes. Finally, we will study the most general matching of cut-and-paste type by considering two regions of the same spacetime.