At Sandia National Laboratories, a 5.5 Megavolt Laser Triggered Gas Switch (LTGS) developed surface flashover within the PMMA envelope. This surface flashover occurs initially during the regular cascade switching process without affecting the switch's electrical performance. However, the switch will pre-fire in the successive shot. The damage to the envelope must be repaired before normal switch operation can resume. A 340 kV system was designed which replicated conditions found in the LTGS. Besides measurement of flashover delay times for various dielectric materials (Lexan, Acrylic, etc.), emphasis was put on characterization of the flashover phenomenology. In addition to voltage and current diagnostics, fast optical imaging of the flashover along with the acquisition of optical emission spectra (200 nm to 700 nm) was employed. This book investigates the two events that occur within the chamber, surface flashover and volume breakdown. Understanding the fundamental physical mechanisms and the extent to which these processes influence discharge behavior is vital to characterizing and modeling electrical breakdown in high pressure SF6.