Some of the latest results on longitudinal polarisation measurements at LHCb are summarised here. The experimental observables are introduced as well as the main frameworks within QCD that are used to study them from a theoretical point of view. Seven analyses are discussed, containing both charmed and charmless $B$-decays. While charmed $B$-decays proceed through tree diagrams at leading order, charmless $B$-decays can proceed through tree, tree and penguin, or pure penguin diagrams.