This chapter will discuss various aspects of operational architectonics methodology for EEG analysis that have been developed over the course of last 17 years in relation to nonstationarity of brain functioning. At first we will detail the peculiarities and evidence for a spatial and temporal nonstationarity in the EEG signal, then we will review a theoretical framework that could integrate existing data with a focus on theoretical advantages provided by an operational architectonics framework, and finally we will describe the experimental results related to methodology. In the last part of the chapter we will outline the application of OA methodology to clinical, pharmacological, cognitive and neurophilosophical studies.