Silicon is an excellent detector material for electromagnetic radiation in the wavelength range of 0.1 to 1000 nm. In the visible spectral range (400-700 nm), external quantum efficiencies approaching 100% are obtained. When combined with the amazing miniaturization capabilities of the semiconductor industry, this fact explains why silicon is the material of choice for very efficient, highly integrated, cost-effective image sensors: In 2007 about one billion image sensors were produced and employed in camera systems. Profiting from the unrelenting progress of semiconductor technology, silicon-based image sensors with astounding performance have been demonstrated, in terms of resolution, pixel size, data rate, sensitivity, time resolution and functionality: 111 million pixels on a single CCD chip were produced, pixels with a period of 1.2 m were fabricated, sustainable image acquisition and readout rates of 4 billion pixels per second were realized, single-photon sensitivity at room temperature and at video rates was achieved, timing resolution of the pixels in lock-in image sensors below 5 ps was obtained, and the processing complexity of ―smart pixels‖ was raised to several ten transistor functions per pixel. The future of semiconductor image sensing lies in the extension of the accessible wavelength range to the infrared spectrum (1.5-10 m), the development of affordable, high-performance X-ray image sensors in particular for the medical energy range (20-120 keV), the realization of sensitive and cost-effective sensors for Terahertz imaging (100-500 m), as well as the integration of an increasing amount of analog and digital functionality on single-chip custom camera systems. The Holy Grail is the ―seeing chip‖, capable of analyzing the contents of a scene and of recognizing individual objects of interest.