In photoredox catalysis, light is absorbed by a photocatalyst to form its excited state, which can then undergo electron transfer processes with organic substrates in the reaction mixture. This typically yields the formation of organic radicals, which can then react further to form new compounds of interest. The use of photoredox catalysis in organic synthesis has grown fast in the past fifteen years, and new synthetic methodologies are continuously proposed. At the same time, the mechanistic investigation of these new reactions has lagged behind. Understanding the reaction mechanism is crucial for further advances in the field of photoredox catalysis, as it can provide useful insights on how to design new protocols in the most efficient way. In this thesis, mechanisms of photoinduced electron transfer and proton-coupled electron transfer (PCET) in systems of relevance for photoredox catalysis are investigated, through spectroscopic measurements (steady-state absorption and emission, femtosecond and nanosecond transient absorption and stopped-flow techniques). In Paper I, the mechanism of halophosphines activation via an Ir-based photocatalyst is explored. Here, back electron transfer is found to be dominant over the productive reaction with the phosphine substrates, which can only react at later timescales. Papers II and III focus on the photoredox activation of O-H bonds with an organic photocatalyst (9-mesityl-10-methylacridinium), in the context of β-scission reactions from alcohols. As the very first steps of the photoredox catalytic cycle are explored, support to the photophysical characterization of the photocatalyst is provided, which has shown complexity and has been controversial in the literature. Evidence for a PCET pathway in the formation of the O-centered radical is shown, which is found to be dominant over other competing pathways. In Paper IV, an NADH (nicotinamide adenine dinucleotide) analogue is used as model system to explore the PCET activation of C-H bonds. In particular, the possibility of accessing the concerted transfer of the electron and the proton (CEPT) to two different acceptors is investigated, and the factors influencing the observed dominance of a stepwise over concerted mechanism are discussed. The results also suggest the symmetric dependence of the rate constant for the concerted mechanism on the driving forces for electron and proton transfer, as opposed to recent reports on C-H bond activation via CEPT.