Stimulation of immune-mediated effector functions via the antibody Fc domain such as complement-dependent cytotoxicity (CDC) or antibody-dependent cell-mediated cytotoxicity (ADCC) are key mechanisms of action (MoA) of many therapeutic antibodies. Functional bioassays are a key asset in the method portfolio used throughout biologics development, characterization and release. The presentation gives an overview of the various Fc effector functions, it shows data on the typically observed assay precision and discusses strategies to increase long term stability. Using ADCC as an example, different assay formats are discussed and their performance characteristics are compared. The presentation also discusses the application of the established structure-function relationships for ADCC to monitor assay stability and to select the most relevant assay type to ensure product consistency and to demonstrate comparability and biosimilarity, respectively.