Scintillators have been used for decades to make ionising radiation visible. Either by direct observation of the light flash produced by the material when it is exposed to radiation, or indirect by use of a photomultiplier tube or photodiode. Despite the enormous amount of commercially available scintillators, the ideal scintillator that combines a high light yield, a high density, a fast decay, and good energy resolution has yet to be found. In the thesis of Van Loef two topics in scintillator research are explored: 1) the search for new inorganic scintillators for gamma ray detection and 2) the systematic research of the scintillation mechanisms in scintillator materials. It was decided to limit the research to materials based on lanthanum, gadolinium and lutetium halides, doped with trivalent cerium. For most materials, the light yield, decay time, and energy resolution were determined. For LaCl3:Ce3+ en LaBr3:Ce3+ a world record energy resolution of about 3% at 662 keV (137Cs) was obtained. Eventually a patent application was filed and granted. During the systematic research of the scintillation mechanisms attention was focussed on the influence of the anion on the scintillation properties of the materials studied. It appears that the speed of energy transfer increases in the series Cl to Br to I. Additionally, it appeared that "self-trapped exciton diffusion" plays an important role in materials based on lanthanum, whereas "binary electron-hole diffusion" is probably dominant in materials based on lutetium.