While 3D printing is still on its way attracting more and more attention from different communities, the concept of 4D printing has just merged and added in a new dimension to widen the application field of 3D printing. In this talk, we summarize the major techniques for 4D printing into the following categories,
I. Self-assembly of elements.
II. Stimulus-induced mismatch in deformation.
III. Compliant mechanism based bistable structure.
IV. Shape memory effect (SME).
Technical details of them are presented, together with their advantages and disadvantages.