Abstract: Locks for security grills used on doors or windows tend to be structurally complicated, and consequently expensive to produce and/or install. A relatively simple lock assembly for use with a security grill includes a box for mounting on one edge of a grill, a slot in one side of the box for opposing the frame of the door or window when the grill is closed, a plate for mounting on the outside of the frame in opposition to the box, whereby the plate completely covers the slot when the grill is closed, a bolt extending through the plate for rotatable mounting in the frame in alignment with the slot, an ear on the outer end of the bolt for insertion through the slot into the box when the grill is moved to the closed position, and a handle on the inner end of the bolt for rotating the latter between a release position in which the ear can be inserted into or removed from the box via the slot and a locked position in which the ear is retained by a projection in the box.