The invention provides a golf club head that includes a weight adjustment system inside of the club head. Preferably, when the club is played, no part of the adjustment system is directly accessible or exposed to an exterior of the club. The club head may include a panel that opens or is removable, or the club head may be disassembled to provide access to the internal weight adjustment system. A weight adjustment system generally includes at least one mechanism by which a mass distribution of the golf club head can be changed.