A folding apparatus for folding sheets of flexible sheet materials, which includes a feeding mechanism (11, 17) for feeding sheets of material, a folding mechanism including a pair of rollers (11, 13) having a nip (23) into which a sheet of material (7) is inserted to create a fold (7b), and an inserter mechanism (21) for inserting a sheet (7) into the nip (23). The inserter mechanism (21) includes knife element (29) having an edge (31) that is arranged to engage a sheet (7) along a designated fold line (7a) and to insert said sheet (7) into the nip (23) to produce a fold (7b) along said fold line (7a). The inserter mechanism (21) is constructed and arranged to insert the sheet (7) into the nip (23) while the sheet (7) is positively engaged by the feeding mechanism (11, 17).