Abstract: An exercise device comprising a core element which may be generally spherical and interchangeable shells or covers which are mounted in an overlying relationship with the core to vary the total weight. The covers may have a two-part hinged construction or be segmented with the segments of each cover being interlocking. In one embodiment, the covers are sized to allow each cover to be mounted or nested one upon another in order to vary the weight. The cover may include two spherical panels having at least an outer wall element. Weight blocks, which can be mounted to the outer wall element or placed in a space between the outer wall element and an inner wall element, allow the mass of the cover to be adjusted. The exercise device may have attachment means to attach it to a bar unit, such as a barbell, or may be used as a free-standing exercise ball.