The injection of sinusoidal current to an electrical grid is nowadays possible with the use of active power filters (APFs).With quasi-steady-state (QSS) nonlinear controllers, these power filters provide low grid-current-harmonic content with simple-control analog implementation. This paper focuses on the design of a QSS nonlinear controller for a single-phase APF. A complete analysis based on the sliding-mode-control theory is given. From the analysis, a set of control-design guidelines is presented to select the gains and parameters of the control scheme. Selected experimental results are reported to validate the theoretical design procedure.