Abstract: Chemiluminescence (CL) and bioluminescence (BL) are natural phenomena which have attracted the attention of mankind since the evolution of life on the earth. The various mechanisms of formation of cold light were explained during this century and CL was established as an analytical technique during the last few decades. The dramatic growth in the quality and quantity of analytical chemiluminometric methods came soon after the introduction of flow injection analysis into the area of CL and the development of successful flow cells. Hence, CL is now considered as a very sensitive analytical technique. A plethora of new chemiluminogenic reactions have been discovered by trial and error and successfully used analytically, but efforts to foresee which molecules participate in CL reactions are continuously made. Over the last few decades some general rules have been modulated and are briefly presented in this paper. A practical example is the CL generated during the action of potassium permanganate on trimetazidine (1-(2,3,4-trimethoxybenzyl)piperazine) which is presented in this paper. The discovery is important since analytical procedures for determining trimetazidine are very limited in number and new methodologies are required for assaying the drug in biological fluids. The analytical procedure which was finally developed allows the determination of trimetazidine within the linear range of 5-1000 ng mL-1. Less known CL techniques which offer great advantages over other analytical methods are also presented. The beginning of the millennium will find CL as a powerful tool for sensitive measurements after miniaturized separation systems and as field analyzer.s. Emphasis should be given to the thorough investigation of the mechanism of the reactions which generate radiation to allow full control of the capabilities of the technique.