Infrared singular factors originating from the QCD factorisation of scattering matrix elements in soft and collinear limits play a prominent role in both organising and computing higher order contributions to hard scattering cross sections. In this talk, I will report on recent work for the evaluation of collinear contributions to QCD resummations of hard-scattering observables. We start from the amplitude factorisation formulae in the collinear limit, and we introduce fully-differential collinear functions that have a process-independent structure. These collinear functions can then be integrated over the appropriate observable-dependent phase space to compute logarithmically-enhanced contributions to the observable of interest. For transverse-momentum dependent observables, we show how collinear functions can be defined without introducing what is known as rapidity divergences in the literature. We present analytic expressions for the collinear functions up to NNLO in perturbation theory.