Chapter 3 covers the fundamental principles of brain activation and measurement, along with measurement design issues. Specific structural and functional brain imaging techniques are discussed, including computed tomography (CT), magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), functional magnetic resonance imaging (fMRI), positron emission tomography (PET), electroencephalogram (EEG and qEEG), real-time functional magnetic resonance imaging (rtfMRI), and near infrared spectroscopy (NIRS). Current methodological limitations of neuroimaging techniques in the study and diagnosis of psychological disorders are explained. The chapter also reviews brain stimulation techniques, including electroconvulsive therapy (ECT), transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS), transcranial electrical stimulation (tES), optogenetics, vagus nerve stimulation (VNS), and deep brain stimulation (DBS). The application of these techniques in the treatment of psychiatric and neurodegenerative disorders is discussed as well in subsequent chapters.