Abstract: A concrete form system comprising a reversibly stackable insulating form blocks each block having a pair of panels separated by two or more ties. Each of the top, bottom and side edges of the panels includes either a tongue or groove so as to engage a corresponding tongue or groove of a panel of another block stacked vertically or horizontally adjacent thereto. The panels of each block are arranged in opposite orientation to each other so as to enable the block to be stackable in either an upside-down or right-side-up orientation.