New laser technology has developed a new dye chemistry! Development of the gallium-arsenic semiconductor laser (diode laser) that emits laser light at 780-830 nm has made possible development of new opto-electronic systems including laser optical recording systems, thermal writing display systems, laser printing systems, and so on. Medical applications of lasers in photodynamic therapy for the treatment of cancer were also developed. In such systems, the infrared absorbing dyes OR dyes) are currently used as effective photoreceivers for diode lasers, and will become the key materials in high technology. At the present time the chemistry of IR dyes is the most important and interesting field in dye chemistry. Laser light can be highly monochromatic, very well collimated, coher­ ent, and, in some cases, extremely powerful. These characteristics make diode lasers a very cheap, convenient, and useful light source for a variety of applications in science and technology. For these purposes, however, IR dyes with special characteristics are required. To develop new IR dyes, it is most important to establish the correlation between the chemical structures of dyes and other characteristics of dyes, such as their absorption spectra. Molecular design of IR dyes predicting the Amax and Emax values by molecular orbital (MO) calculations is now possible even by using a personal computer, and many types of new IR dyes have been demonstrated. Also, new opto-electronic systems using IR dyes as photoreceivers have been reported recently.