Carbon nitride (CN_x) is an emerging photocatalyst with the potential to efficiently produce solar fuels. CN_x heterojunctions often show significant photocatalytic activity improvements. We review the charge carrier dynamics in a range of CN_x heterojunctions including carbon-based material, black phosphorus, Ru complexes, molybdenum sulphide and metal phosphides. Time resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) were the most common techniques employed for experimental charge carrier dynamics measurements. The low photoluminescence quantum yield of CN_x appeared to limit the depth of conclusions from TRPL, with both lengthening and shortening of the TRPL lifetimes observed and attributed to enhanced charge separation. Overall, the charge carrier dynamics studies often showed a relative lifetime change of ∼2-fold and an activity improvement of >10-fold. We highlight the need for the use of a wider range of techniques to monitor the charge carrier dynamics for conclusive determination of photophysics–activity relationships and elucidation of improvement mechanisms.