A grenade firing mechanism 12 has a body 18 defining an internal chamber 32 in which a firing pin structure 40 is located. The firing pin structure is actuated by an inertia toggle 64 having a first end contained within the chamber and a second end region 72 which projects from the body. A safety lever 16 is releasably mounted to the second end region 72 of the inertia toggle in an operative position to prevent the inertia toggle moving to actuate the firing pin structure. A lever spring 104 is operative to eject the lever from the inertia toggle allow the firing pin structure to be actuated when the grenade is deployed. The lever 16 carries an abutment pin 120 which extends through the body to engage the firing pin structure 40 to inhibit the firing pin structure moving in a firing direction when the lever is in its operative position. The inertia toggle may have a convex abutment surface which engages the firing pin structure. The firing mechanism is particularly suited to a sound flash distraction grenade.