The neurons can transform different spatiotemporal patterns of synaptic inputs to the action potential sequences with high temporal precision. This flexible and reliable information coding strategy plays a crucial role in the process by which the nervous system generates the specific activity patterns required by dynamical situation or specific task. The initiation of an action potential follows an all-or-none principle. When the depolarization of membrane potential exceeds a threshold value, the neuron fires an action potential. The action potential threshold is highly variable within and between cells, and its specific dynamics depends on the stimulus input and firing history. In particular, the spike threshold is sensitive to the membrane voltage changes preceding the action potential. Two primary biophysical mechanisms for such state dependence of the spike threshold are Na⁺ inactivation and K⁺ activation. In most neurons, the action potentials are initiated in the axon initial segment, and the threshold variability at this site is the crucial factor that determines how neurons transfer spatiotemporal information. However, the action potentials in electrophysiological experiments are recorded in the cell body or proximal dendrite. The threshold variability at these sites is higher than that in the axon initial segment, which mainly arises from the backpropagation of axonal action potentials. Based on somatic recordings, it is shown that the spike threshold dynamics determines the transformation principle of spatiotemporal information in the neurons, which enhances the temporal coding, feature selectivity, gain modulation, and coincidence detection. In this paper, we first introduce the conception of spike threshold and its calculation methods. Then, we present an exhaustive review on the main findings of the spike threshold variability and its origins in recent years, and mainly discuss the significance of spike threshold variability for neuronal coding. Finally, we raise several key issues on the spike threshold that need to be addressed in the future.