This paper presents a model for nonperturbative nonlinear optics involving continuum states, which helps optimize and control high-order nonlinear susceptibility. It clarifies the connection between above-threshold ionization and harmonic generation. The model shows that there is not a one-to-one correspondence between above-threshold ionization peaks and harmonic emission. It also explains why the maximum energy of harmonic emission differs from that of above-threshold ionization. The strength of harmonic emission increases approximately linearly with the ionization rate. The paper applies concepts from plasma physics to strong-field atomic physics. Since the products of atomic ionization are the basic constituents of plasmas, plasma methods are applicable. High-field atomic physics and plasma physics are becoming increasingly intertwined. The essential point is that an atom undergoing multiphoton ionization does not immediately become a well-separated electron and ion. Rather, there is a significant probability of finding the electron in the vicinity of the ion for one or more laser periods. The paper extends the quasistatic model of multiphoton ionization to include the electron interaction with the ion. This allows quantitative predictions of double ionization, hot above-threshold ionization, and high-harmonic generation. The paper presents a unified approach to these phenomena using one free parameter that is severely constrained. The quasistatic model uses a dual procedure. First, the probability of ionization is determined using tunnel ionization models. Second, classical mechanics is used to describe the evolution of an electron wave packet. The model is validated by comparing with experimental results for above-threshold ionization of helium. The results show excellent agreement with experimental data. The paper discusses the implications of the electron-ion interaction for linearly polarized light. It shows that half of the electrons ionized by linearly polarized light pass the position of the ion once during the first laser period. The other electrons never pass the ion. The probability of finding an electron passing the ion with energy E is determined by equations derived from the model. The paper also discusses the emission of light due to electron-ion interaction. The emission is calculated from the expectation value of the dipole operator. The results show that the harmonic spectrum calculated using the model agrees well with experimental results. The paper concludes that the transverse spread of the electron wave function is an important parameter that can be inferred from experiments using elliptically polarized light. Such experiments and their consequences will form an important new direction in strong-field atomic physics. Future experiments using linearly polarized light to study harmonic generation, double ionization, or above-threshold ionization will be most valuable if the polarization of the laser pulse is known precisely.This paper presents a model for nonperturbative nonlinear optics involving continuum states, which helps optimize and control high-order nonlinear susceptibility. It clarifies the connection between above-threshold ionization and harmonic generation. The model shows that there is not a one-to-one correspondence between above-threshold ionization peaks and harmonic emission. It also explains why the maximum energy of harmonic emission differs from that of above-threshold ionization. The strength of harmonic emission increases approximately linearly with the ionization rate. The paper applies concepts from plasma physics to strong-field atomic physics. Since the products of atomic ionization are the basic constituents of plasmas, plasma methods are applicable. High-field atomic physics and plasma physics are becoming increasingly intertwined. The essential point is that an atom undergoing multiphoton ionization does not immediately become a well-separated electron and ion. Rather, there is a significant probability of finding the electron in the vicinity of the ion for one or more laser periods. The paper extends the quasistatic model of multiphoton ionization to include the electron interaction with the ion. This allows quantitative predictions of double ionization, hot above-threshold ionization, and high-harmonic generation. The paper presents a unified approach to these phenomena using one free parameter that is severely constrained. The quasistatic model uses a dual procedure. First, the probability of ionization is determined using tunnel ionization models. Second, classical mechanics is used to describe the evolution of an electron wave packet. The model is validated by comparing with experimental results for above-threshold ionization of helium. The results show excellent agreement with experimental data. The paper discusses the implications of the electron-ion interaction for linearly polarized light. It shows that half of the electrons ionized by linearly polarized light pass the position of the ion once during the first laser period. The other electrons never pass the ion. The probability of finding an electron passing the ion with energy E is determined by equations derived from the model. The paper also discusses the emission of light due to electron-ion interaction. The emission is calculated from the expectation value of the dipole operator. The results show that the harmonic spectrum calculated using the model agrees well with experimental results. The paper concludes that the transverse spread of the electron wave function is an important parameter that can be inferred from experiments using elliptically polarized light. Such experiments and their consequences will form an important new direction in strong-field atomic physics. Future experiments using linearly polarized light to study harmonic generation, double ionization, or above-threshold ionization will be most valuable if the polarization of the laser pulse is known precisely.