The effects of unbalances, harmonics, and interharmonics on PLL systems are analyzed. After a brief review of some typical schemes for three-phase and single-phase PLLs, an analytical assessment of the phase angle and frequency errors for PLLs in the presence of disturbances is performed to obtain simple analytical formulas. Numerous case studies for the significant conditions and a possible application to the design of PLLs or to the compliance tests for PLL standards are discussed. Finally, the results of laboratory experiments on a power quality measurement instrument are presented.