AM plus post-processing is as old as AM, but the aim of the post-processing has always been to improve properties. Four-dimensional printing (4DP) is a relatively new technique that is also AM plus post-processing, but the aim of the post-processing is to change the shape rather than to change properties. It does not mean that there will be no property changes in 4DP; properties will change, but changing the property is not the primary aim of 4DP. If there is no shape change in 4DP, it will not be called 4DP even if there is a change in properties. 4DP, as the name suggests, has four dimensions—three dimensions for space and an additional dimension for time. While an object of three dimension is acquired by 3DP (or AM), neither the object nor its shape is final until the application of the fourth dimension is exercised. 4DP is a generic process that starts from various AM processes such as digital light processing, stereolithography, powder bed fusion, inkjet printing, fused deposition modeling, etc., and is thus not a variant of AM processes but is an application of AM. 4DP has been defined as “additive manufacturing of objects able to self-transform, in form or function when are exposed to a predetermined stimulus, including osmotic pressure, heat, current, ultraviolet, or other energy sources.”