The accumulation of insoluble neurofibrillary tangles, composed of hyperphosphorylated tau, is one of the main pathological hallmarks of tauopathies. Growing evidence indicates that the release and re-uptake of pathogenic tau seeds mediates the spread of tau pathology along living and intact neuronal networks via ‘prion-like’ mechanisms. Moreover, there is evidence showing that neuronal activity can regulate tau secretion and accelerate cell-to-cell propagation of tau pathology in vitro and in vivo. What molecular mechanisms drive and mediate activity-dependent secretion of physiological and pathological tau remains to be investigated. Sensitive biosensors that are able to monitor the release and re-uptake of tau in connected neuronal networks under different conditions are currently limited. Split nanoluciferase (NLuc) complementation reporters, which are composed of two subunits, a large bit (LgBiT; 17.6kDa) and a smaller bit (HiBiT; 11 amino acid) is a highly-sensitive bioluminescent biosensor that can be used to monitor cell-to-cell transfer of tau in different conditions. Structural complementation of the split HiBiT and LgBiT reporters re-constitutes the NLuc enzyme, which generates a bright luminescent signal. Tagging tau with these reporters will allow the mechanisms of tau release and re-uptake to be closely dissected, under pathological and physiological conditions. A greater understanding of the mechanism involved in the spread of tau pathology will help in the development of effective treatments in the future.