This paper by P.B. Corkum discusses the plasma perspective on strong-field multiphoton ionization, focusing on the electron-ion interaction and its consequences. The author introduces a model for nonperturbative nonlinear optics involving continuum states, which helps optimize and control high-order nonlinear susceptibility. The model clarifies the connection between above-threshold ionization and harmonic generation, showing that there is no one-to-one correspondence between the two processes. The strength of harmonic emission increases linearly with the ionization rate. The paper applies plasma physics concepts to strong-field atomic physics, emphasizing that an atom undergoing multiphoton ionization does not immediately separate into a well-defined electron and ion. Instead, there is a significant probability of finding the electron near the ion for one or more laser periods. The quasistatic model, which includes the electron-ion interaction, quantitatively predicts double ionization, hot above-threshold ionization, and high-harmonic generation. The model uses tunneling ionization models to determine the probability of ionization as a function of the laser electric field and classical mechanics to describe the electron wave packet's evolution. The paper discusses the implications of the electron-ion interaction for correlated two-electron ejection and harmonic generation. It also highlights the importance of the transverse spread of the electron wave function and the laser polarization in controlling these phenomena. The author concludes that the transverse spread of the electron wave function is a crucial parameter that can be inferred from experiments using elliptically polarized light. This parameter will be important for future experiments studying harmonic generation, double ionization, and above-threshold ionization using linearly polarized light.This paper by P.B. Corkum discusses the plasma perspective on strong-field multiphoton ionization, focusing on the electron-ion interaction and its consequences. The author introduces a model for nonperturbative nonlinear optics involving continuum states, which helps optimize and control high-order nonlinear susceptibility. The model clarifies the connection between above-threshold ionization and harmonic generation, showing that there is no one-to-one correspondence between the two processes. The strength of harmonic emission increases linearly with the ionization rate. The paper applies plasma physics concepts to strong-field atomic physics, emphasizing that an atom undergoing multiphoton ionization does not immediately separate into a well-defined electron and ion. Instead, there is a significant probability of finding the electron near the ion for one or more laser periods. The quasistatic model, which includes the electron-ion interaction, quantitatively predicts double ionization, hot above-threshold ionization, and high-harmonic generation. The model uses tunneling ionization models to determine the probability of ionization as a function of the laser electric field and classical mechanics to describe the electron wave packet's evolution. The paper discusses the implications of the electron-ion interaction for correlated two-electron ejection and harmonic generation. It also highlights the importance of the transverse spread of the electron wave function and the laser polarization in controlling these phenomena. The author concludes that the transverse spread of the electron wave function is a crucial parameter that can be inferred from experiments using elliptically polarized light. This parameter will be important for future experiments studying harmonic generation, double ionization, and above-threshold ionization using linearly polarized light.