Artifacts are EEG waveforms not generated by the brain. The main purpose of recognizing artifacts is to avoid mistaking them from seizures. They may originate from other body organs (internal) or environmental sources (external). Common internal artifacts include ocular (eye movement), glossokinetic (tongue movement), cardiac (ECG), myogenic (muscle activity), or sweat-sway artifact (skin). Common sources of external artifact include electrodes, ventilators, suction devices, bed percussion, chest compression, and various medical devices. Many commonly used medications are associated with EEG changes. These include excessive alpha and beta activity (e.g., barbiturates and benzodiazepines), theta and delta slowing (antiseizure and psychotropic medications), spike and sharp waves (clozapine), and rhythmic and/or periodic patterns (cefepime). EEG patterns of common intravenous and inhalational anesthetic agents are also described in this chapter.