Evaluation of total complement function in human serum is an essential component of laboratory diagnostics of the human complement system. During recent years, established hemolytic assays for classical pathway and alternative pathway function, CH50 and AP50 assays, respectively, have been replaced in many diagnostic laboratories by ELISA assays. Next to an improved standardization, this assay platform also allows for functional analysis of the lectin pathway of complement. The present chapter describes the methodology of ELISA assays for assessment of the classical pathway, the alternative pathway, the MBL-dependent lectin pathway, and the Ficolin-3-dependent lectin pathway of complement in clinical laboratory diagnostics.