Remote detection of radioactive materials is extremely challenging, yet it is important to realize the technique for safe usage of radioactive materials. Gamma rays are the most far distant penetrating photons that are involved with the radiation decay process. Herein, we overview the gamma-ray detection techniques that are material-based and vacuum tube-based. A muon detector is also reviewed as a radioactive material imager. We overview versatile detectors that are currently being widely used and new concepts that may pave the way for promising remote detectability up to several kilometers.