The paper reviews methods of investigation into the structure and functional role of working memory (WM). The methodology of WM studies are comprehensively analyzed from the perspective of two main theoretical approaches. Firstly, Alan Baddeley's multiple-systems model of WM serves as a ground for the review and analysis of different methodologies aimed at determining the capacity and functions of two slave subsystems (phonological loop and visual sketchpad) dedicated to storing and processing verbal and visual information, respectively. The advantages and limits of these are concerned as well. Secondly, Cowan's single-system model of WM based on activation mechanism is analyzed. The authors also present methods aimed at investigating control and executive functions of the WM system.