Correlations between two hadrons in a jet and their modification due to the jet traversing a dense medium are systematically studied within the generalised factorization and higher twist formalism of perturbative QCD. Two particle correlations are understood within the formalism of a universal dihadron fragmentation function. Modifications to the dihadron fragmentation function arise from the multiple scattering of the struck parton off soft gluons and due to energy loss encountered in the radiation of a gluon. Comparisons with the data from the HERMES experiment where the initial energy of the struck parton is measured are presented. This formalism is then generalized to estimate modifications to triggered distributions in hot and dense matter created in a heavy-ion collision. The effect of triggering on the energy of the initiating parton, and the ensuing associated multiplicity are discussed. Comparisons with the data from the STAR and PHENIX experiments will be presented.