Time-resolved optical spectroscopy is the perfect method to study photo-induced dynamics of excited particles and quasiparticles. I will introduce a simple, general approach based on intensity variation that allows separation of multi-particle dynamics in time-resolved methods. Exemplary applications include disentangling of carrier recombination mechanisms in nanocrystals, as well as using exciton--exciton annihilation to track diffusive transport. FInally, I will present a novel type of so-called action spectroscopy with dual signal detection that measures ultrafast dynamics via both radiative and non-radiative excitation recombination.