Apparatus for folding sheet blanks (1), e.g. for boxes and trays from carton or corrugated cardboard. It includes press means (6, 7) attached to at least one endless conveying means (10). The press means are intended to coact with selected parts (2, 3) of the blanks for folding them up or down from the plane of the blank during their passage through the apparatus. The blanks are arranged to be conveyed through the apparatus at the same speed as that at which the conveying means is driven. The latter is guided in a helical path by an elongate, rigid structure (8, 9) forming continuous support for the conveying means. During the entire folding operation the operative ones of the press means attached to the conveying means press against the same area on the part of the blank with which they coact.