Causal set theory, originally introduced by Rafael Sorkin, is a model of spacetime as a partially ordered set: an element of a set corresponds to a point in spacetime, while partial ordering corresponds to lightcone causal relation. There is no coordinate system: all of the geometry is to be deduced from partial ordering alone. Consequently, one has to rewrite Lagrangians in quantum field theory in such a way that would avoid derivative signs or anything else with Lorentz index. In my talk I will discuss some of the ways of doing so (both the ones introduced by myself and by others).