The complement system is a central part of the innate immune system. It defends the human body against infections and helps with the clearance of apoptotic particles and cellular debris. The importance of the complement system in physiology is reflected by autoreactive diseases that occur due to loss of functions of complement regulators as identified in age-related macular degeneration or gain of functions in complement convertases like C3 glomerulopathy. The chapter aims to provide methods to study complement regulation on a molecular level. Here we describe a set of in vitro assays, the combined techniques of ELISA and immunoblotting, to determine complement activation and regulation on surfaces. The methods allow to follow part of the complement activation cascade and to determine the activity of complement regulators like factor H.