Iron-type golf club heads are disclosed having a heel portion, a sole portion, a toe portion, a top-line portion, a front portion, a rear portion, and a striking face. The iron-type golf club heads include a flexible boundary structure (“FBS”) that is provided at one or more locations on the club head. The flexible boundary structure may comprise, in several embodiments, a slot, a channel, a gap, a thinned or weakened region, or other structure that enhances the capability of an adjacent or related portion of the golf club head to flex or deflect and to thereby provide a desired improvement in the performance of the golf club head.