<p>Wheels have been fundamental tools in human civilization since ancient times. Currently, they are extensively used in mobile robotics owing to their simplified systems and stability on flat surfaces. However, conventional wheels face limitations when operating in uneven and unpredictable environments, where they may slip or struggle to overcome obstacles larger than their radii. Transformable wheels offer an innovative solution to address these challenges. Despite their potential, the transformable wheel field is fragmented by diverse terms describing wheel transformations, making it difficult to integrate knowledge and compare the obstacle-overcoming abilities of different wheel designs, which directly impact mobile robot task performance. In this review, we propose performance indices aligned with the mechanical complexity of transformation and analyze various transformation mechanisms to improve our understanding of this technology for mobile robots.</p>