The early steps with 2-dimensional semiconductor detectors for particle physics experiments are briefly described. A comparison is made between monolithic devices, especially the CCD, and hybrid detectors, which combine a semiconductor sensor matrix with a separate ASIC in advanced CMOS technology. There is only a fragmentary treatment of the exploitation of the pixelated silicon systems in the LHC experiments, although these have become essential for tracking and vertexing in the high particle density in LHC. Some applications in other fields are mentioned. The difference is pointed out between the single quantum processing in these imagers, and the usual imagers for visible radiation. A few thoughts are developed in view of future pixel detector developments.