A combined modelling and experimental investigation of the turbulent jet ignition system has been undertaken in a specially-designed flow rig (constant-pressure chamber) with the aim to identify the key physical processes occurring in this promising technology for natural gas engines. Performing this research at atmospheric pressures allows optical access that is difficult to achieve in a realistic engine environment. Various aspects such as: the nature of the fluid escaping the pre-chamber (i.e. unburnt, partially-burnt, fully-burnt); the probability of ignition of the mixture in the main chamber; the effects of geometrical parameters such as nozzle diameter and shape and chamber length; and the effect of flow in the main chamber and of mixture strength in both chambers, are systematically studied. Diagnostics include schlieren and OH* and CH* chemiluminescence imaging, and OH and CH2O planar laser-induced fluorescence. The data interpretation is assisted by one-dimensional strained flame detailed chemistry simulations and a zero-dimensional (zone) model that provide some reference data on ignition delay times and flow rate out of the nozzle as a function of time. The results demonstrate the stochasticity of the ignition processes, the location of the ignition initiation, and provide insights into the reasons the ignition may be successful or unsuccessful.