This paper presents the Astrophysical Plasma Emission Code (APEC) and the Astrophysical Plasma Emission Database (APED), which are used to calculate spectral models for hot plasmas. APEC uses atomic data from APED to compute line and continuum emissivities for collisionally-ionized plasmas. APED contains collisional and radiative rates, recombination cross sections, dielectric recombination rates, and satellite line wavelengths for a wide range of ions. The authors compare APEC results with other plasma codes and test the sensitivity of their results to the number of levels included in the models. They find that dielectric recombination with hydrogen-like ions into high principal quantum numbers affects some helium-like line ratios from low-lying transitions. The paper discusses the effects of changing the underlying atomic data on hydrogen-like (H-like) and helium-like (He-like) oxygen ions. The authors use the H-like Lyβ/Lyα ratio and the He-like G ratio (F+I)/R as diagnostic tools. They compare APEC results with other codes and find that the APEC results agree with the Bethe limit for the Lyβ/Lyα ratio in the high-temperature limit. They also find that the He-like G ratio is affected by dielectronic recombination rates. The authors test the accuracy of their models by running APEC with different levels of oxygen ions and find that including levels up to n=10 improves the convergence of the results. The paper concludes that the accuracy of the underlying atomic data is crucial for the accuracy of the model line ratios. The authors also note that the methods described in this paper can be used to test the importance of detailed treatment of recombination and cascades in diagnostic line ratios. The paper highlights the importance of accurate atomic data for the analysis of X-ray spectra from high-resolution X-ray spectrometers on the Chandra and XMM-Newton telescopes.This paper presents the Astrophysical Plasma Emission Code (APEC) and the Astrophysical Plasma Emission Database (APED), which are used to calculate spectral models for hot plasmas. APEC uses atomic data from APED to compute line and continuum emissivities for collisionally-ionized plasmas. APED contains collisional and radiative rates, recombination cross sections, dielectric recombination rates, and satellite line wavelengths for a wide range of ions. The authors compare APEC results with other plasma codes and test the sensitivity of their results to the number of levels included in the models. They find that dielectric recombination with hydrogen-like ions into high principal quantum numbers affects some helium-like line ratios from low-lying transitions. The paper discusses the effects of changing the underlying atomic data on hydrogen-like (H-like) and helium-like (He-like) oxygen ions. The authors use the H-like Lyβ/Lyα ratio and the He-like G ratio (F+I)/R as diagnostic tools. They compare APEC results with other codes and find that the APEC results agree with the Bethe limit for the Lyβ/Lyα ratio in the high-temperature limit. They also find that the He-like G ratio is affected by dielectronic recombination rates. The authors test the accuracy of their models by running APEC with different levels of oxygen ions and find that including levels up to n=10 improves the convergence of the results. The paper concludes that the accuracy of the underlying atomic data is crucial for the accuracy of the model line ratios. The authors also note that the methods described in this paper can be used to test the importance of detailed treatment of recombination and cascades in diagnostic line ratios. The paper highlights the importance of accurate atomic data for the analysis of X-ray spectra from high-resolution X-ray spectrometers on the Chandra and XMM-Newton telescopes.