Commercial systems for in situ quantification can be categorized as monochromatic slit scanning densitometers, diode-array scanners, and image analyzers (charged-coupled device cameras and flat bed scanners). Specific detectors for radioisotopes and interfaces for direct evaluation by spectrosocopic instruments (mass spectrometry and infrared and Raman spectroscopy) are also available. In recent years there has been a growing interest in biological detection using, among others, various luminescence reactions and conditions that facilitate instrumental detection. This approach complements a long history of the use of in situ chemical reactions for selective and sensitive detection or visualization of thin-layer chromatograms (see Section 4).