The inspection and maintenance of small pipes with diameter of 1 in. or below are important tasks given the crucial role of these pipes in various applications, including power plants, nuclear facilities, and underwater systems. Miniature pipe robots (MPRs) are moving vehicles that could take the inspection sensors to the required place inside the pipelines. In this paper, the state-of-the-art researches on MPRs are reviewed. Important design considerations are presented while introducing existing successful designs. The future work of MPRs is also outlined.