In the present paper the results of the study of semiconductor and optical properties of GaSe crystals important for their applications in nonlinear optics of terahertz and mid-IR ranges are reported as well as influence of doping with isovalent chemical elements on them. The performed first-principles calculations of charge neutrality level (CNL) in GaSe have shown its position at 0.8 eV above the top of the valence band. The location of CNL in the lower part of the band gap can explain GaSe intrinsic p-type of conductivity. In this case it is necessary to radically improve the growth technology to obtain GaSe crystals with low free carrier concentrations. Extraordinarily large birefringence of GaSe B~0.8 in terahertz range has been measured directly. In order to study the potential efficiency of application of doped GaSe crystals for terahertz generation and detection their study using terahertz time domain spectroscopy (THz-TDS) setup has been performed. According to the obtained results doped GaSe crystals are slightly less efficient for terahertz detection and generation in the frequency range 0.2-3.2 THz via optical rectification of laser pulses with λ=790 nm and τ=80 fs. On the other hand doping of GaSe with In, Al, S, Te leads to 2-3 times increase of the microhardness and the doped crystals become suitable for the mechanical treatment.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.