This chapter studies the impact of wind and solar generation on frequency quality, particularly long-term frequency quality. It does so based on operational data from the AIPS and through a correlation analysis to determine whether the penetration of wind and solar generation and the magnitude of system frequency fluctuations show any statistical correlation. This analysis is important for using TSOs as it is not yet clear, especially in large interconnected power systems, if wind and solar PV negatively impact the quality of system frequency. To answer these questions, we quantify, using Pearson correlation coefficient and p-value, the correlation between wind (Sect. 11.1) and solar PV (Sect. 11.2) penetration and frequency fluctuations.