Viewing psychiatric disorders or symptoms as outcomes of brain dysfunction emphasizes the necessity of objectively assessing brain function for a deeper understanding of their pathophysiology. Moreover, to measure the brain’s functional abnormalities underlying the dynamically changing mental states and symptoms, EEG and MEG with high temporal resolution are optimal. Research employing EEG/MEG to investigate brain function in relation to psychiatric disorders via classical task-evoked ERP/ERF studies has produced substantial findings. Nevertheless, recent progress in equipment digitalization and analytical methodology development has facilitated the quantification of neural oscillations, inherent rhythmic periodic activities critical for sustaining complex brain functions. It is evident that these oscillations play a role in the pathophysiology of psychiatric disorders, such as schizophrenia and bipolar disorder. In this chapter, we provide an overview of previous research using EEG/MEG in major psychiatric disorders, along with the latest findings and future prospects.