Single-walled carbon nanotubes (SWCNTs) are filled with metals. This is very useful filler for investigation of the electronic properties, future applications in optoelectronics. This is very important field of utilization of work time, and the filled carbon nanotubes are very clean, and not toxic for human, and environment. The filling of SWCNTs with such metals as silver, copper, europium, and intercalation with potassium, sodium was achieved. The iron filling was also shown. The investigation of magnetic, electronic, electrical, thermal, optoelectronic properties of filled SWCNTs was performed. Silver and copper filling was performed in the solution, using salts with subsequent thermal treatment. The intercalation with metals is performed with the gas phase method. Different methods can be used for the filling of carbon nanotubes with metals, which are discussed here. Applications of metal filled carbon nanotubes include the fields of nanoelectronics as field effect transistor components, components of solar cells, light emitters, thermoelectric power generators with partial filling with metal, catalysts for chemical reactions, sensors for gases and bioobjects, biomedical theranostic agents for bioimaging and delivery, spintronics devices, magnetic storage, magnetic recording with high coercive force, electrochemical energy storage devices, capacitors and supercapacitors. These applications can be realized with controlled filling processes, filling morphology, filling ratio. Therefore, the control of chemical processes of encapsulation is important for all applications.