Phase response curve (PRC) is an extremely useful tool for studying the response of oscillatory systems, e.g. neurons, to sparse or weak stimulation. Here we develop a framework for studying the response to a series of pulses which are frequent or/and strong so that the standard PRC fails. We show that in this case, the phase shift caused by each pulse depends on the history of several previous pulses. We call the corresponding function which measures this shift the phase response function (PRF). As a result of the introduction of the PRF, a variety of oscillatory systems with pulse interaction can be reduced to phase systems. The main assumption of the classical PRC model, i.e. that the effect of the stimulus vanishes before the next one arrives, is no longer a restriction in our approach. However, as a result of the phase reduction, the system acquires memory, which is not just a technical nuisance but an intrinsic property relevant to strong stimulation. We illustrate the PRF approach by its application to various systems, such as Morris-Lecar, Hodgkin-Huxley neuron models, and others. We show that the PRF allows predicting the dynamics of forced and coupled oscillators even when the PRC fails. Thus, the PRF provides an effective tool that may be used for simulation of neural, chemical, optic oscillators, etc.A variety of physical, chemical, biological, and other systems exhibit periodic behaviors.The state of such a system can be naturally determined by its phase [1], that is, the single variable indicating the position of the system within its cycle. The concept of the phase proved to be exceptionally useful for the study of driven and coupled oscillators [1][2][3].In order to describe the response of oscillators to an external force or coupling the so- called phase response curve (PRC) is widely used. The PRC defines the oscillator's response to a single short stimulus (pulse). The PRC can be calculated numerically or measured experimentally for oscillatory systems of different origin [4]. These properties made it a useful tool for the study of forced or coupled oscillators [5][6][7][8][9][10][11], and it is especially effective in neuroscience where the interactions are mediated by pulses. If the pulse arrivals are separated by sufficiently long time intervals, the transient caused by a pulse vanishes before the next one comes. From the theoretical point of view, it means that the system returns to the vicinity of its stable limit cycle before the next pulse arrives, see Fig. 1(a). In this case the effect of each pulse can be described by the classical PRC Z(ϕ), which determines the resulting phase shift given that the pulse arrived at the phase ϕ. Another case when the PRCs are useful is when the forcing is continuous in time but weak ( Fig. 1(b)). In this case the system remains close to the limit cycle, and the phase dynamics can be described by the so-called infinitesimal phase response curve [12].Therefore, the PRC-based approach is applicable for either weak or sparse stimulation.However, in many re...