In the oil, gas, and water industries, in-pipe robots have shown promising solutions to reach the inaccessible parts of pipeline networks for condition assessment, leak detection, and fluid quality monitoring. However, the pipeline networks have different sizes and configurations, and there are long lines of pipes that make the operation of in-pipe robots challenging. In this paper, we first categorize the mechanical systems of in-pipe robots and classify them. We then define four missions

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... rmed by the in-pipe robots: localization, mapping, navigation, and inspection. Next, we present the core methods used for each mission and list the studies that use the presented methods and the way they are implemented. Since image processing approach is a common way to accomplish task(s) for an in-pipe mission, the team decided to provide a comprehensive categorization of image processing techniques and the required tools. We also list the sensors used in in-pipe robots which would help the researchers select the appropriate sensor or sensor module for a specific technique.