This research clarifies the most important principles under which wall-climbing robots operate. Two important aspects must be examined in the study of this type of robot. The first aspect is the adhesion method, which has been analysed extensively over the years with multiple approaches. The second aspect is the movement method, which may or may not be integrated with the adhesion method. Research published in the last two decades has divided these robots into five main types and another subtype, each of which provides high efficiency in a specific environment and application. However, the efficiency decreases in others. For example, a robot with suction cups needs a smooth surface, a robot with ducted fans requires a surface that is free of holes, and a robot with magnetic adhesion needs a metal surface. Two robots, one utilising bioinspired adhesion and the other relying on rope and/or rail grabs, function on different surfaces and perform separate applications. This study determines the most important and effective designs, along with their strengths and weaknesses. The suitable applications and environments of these robot are also presented.