The concept of 4D printing is its formation of complex three-dimensional structures that have the ability to adopt different shapes and forms when subjected to different environmental stimuli. A few researchers simply view 4D printing as an extended technique of 3D printing or additive manufacturing with the added constraint of time. However, the unique shape change mechanism exhibited in this process is a combination of shape programming and the usage of smart active materials mostly polymers. This review article highlights the various smart materials, activation mechanisms and the shape-changing techniques employed in the 4D printing process. The potential of the shape-changing structures and their current applications in various biomedical and engineering fields is also explored. The article aims to emphasize the potential and viability of 4D printing and focused on providing an in-depth insight into the 4D printing process. © 2019 Elsevier Ltd