Power plants require periodical inspections to control their state. To ensure a safe operation, parts that could fail before the next inspection are repaired or replaced, since a forced outage due to a failure can cost up to millions of dollars per day. Non-Destructive Testing (NDT) methods are used to detect different defects that could occur, such as cracks, thinning, corrosion or pitting. Some parts are inspected directly in situ, but may be difficult to access; these can require opening access holes or building scaffoldings. Other parts are disassembled and inspected in workshops, when the required inspection tools cannot be moved. In this thesis, we developed innovative miniature mobile systems able to move within these small and complex installations and inspect them. Bringing sensors to difficult-to-access places using climbing robots can reduce the inspection time and costs, because some dismantling or scaffolding can be eliminated. New miniature sensors can help to inspect complex parts without disassembling them, and reduce the inspection costs, as well. To perform such inspections, miniature mobile systems require a high mobility and keen sensing capabilities. The following approach was used to develop these systems. First, different innovative climbing robots are developed. They use magnetic adhesion, as most structures are made of ferromagnetic steel. Then, vision is embedded in some of the robots. Performing visual inspections becomes thus possible, as well as controlling the robots remotely, without viewing them. Finally, non-visual NDT sensors are developed and embedded in some of the robots, allowing them to detect defects that simple vision cannot detect. Achieving the miniaturization of the developed systems requires strong system integration during these three steps. A set of examples for the different steps has been designed, implemented and tested to illustrate this approach. The Tripillars robots, for instance, use caterpillars, and are able to climb on surfaces of any inclination and to pass inner angles. The Cy-mag3Ds robots use an innovative magnetic wheel concept, and are able to climb on surfaces of any inclination and to pass inner angles, outer angles and surface flips. The Tubulos robots move in tubes of 25 mm diameter at any inclination. All robots embed the required electronics, actuators, sensors and energy to be controlled remotely by the user. Wireless transmission of the commands signals allows the systems to maintain their full mobility without disturbing cables. Integrating Hall sensors near the magnetic systems allows them to measure the adhesion force. This information improves the security of the robots, since when the adhesion force becomes low, the robots can be stopped before they fall. The Tubulo II uses Magnetic Switchable Devices (MSDs) for adhesion. An MSD is composed of a ferromagnetic stator and one or more moving magnets; it has the advantage of requiring only a low force to switch on or off a high adhesion force. MSDs have the advantage of being easy to clean of the magnetic dust that is present in most real environments and that sticks strongly to magnetic systems. As an additional step toward inspection, a camera is embedded on the Cy-mag3D II and the Tubulos. It allows these robots to inspect visually the structures the robots move in, and to control them remotely. The perspective of a climbing robot in an unknown environment is often not enough to give the user a sense of its scale, and to move efficiently in it. A distance sensor is designed and embedded on the Cy-mag3D II, which increases the user's perception of the environment substantially; Finally, an innovative miniature Magnetic Particle Inspection (MPI) system was developed to inspect turbine blades without disassembling them. An MSD is used to perform the required magnetization. The system can automatically inspect a flat surface, performing all the required steps of MPI: magnetize, spray magnetic particles, record images under UV light and demagnetize. Thanks to the strong integration and miniaturization, the system can potentially inspect complex parts such as steam turbines.