Gun muzzle blast and flash phenomena are of importance since they are associated with the formation of large overpressures and intense muzzle flash. About 30% of the chemical energy released from the propellant used in a conventional gun is converted into kinetic energy of the projectile. The remaining energy is mainly contained in the propellant gas-particle mixture which escapes from the muzzle of the gun in a few milliseconds. The sudden discharge produces a blast wave because of the rapid displacement of air originally surrounding the gun. In addition, these gases are generally fuel-rich and mix with air turbulently entrained from the surroundings. Combustion of this mixture causes gun muzzle flash, usually associated with the formation of a secondary blast wave. The design of solid propellant charges, gun performance, muzzle attachments and chemical flash suppressants is guided by the need to keep the above hazards to safe limits. In this paper, blast and flash phenomena are characterized using data of recent investigations and showing illustrative examples of their development.