Introduction Versatile and efficient sources of blue light are of fundamental importance for various applications including optical data storage [1,2], ophthalmologic or other medical techniques [3], and laser displays [4 to 6]. In spite of the rapid development of blue laser diodes [7,8] and concurrent physical principles like optical upconversion[9], the optical second-harmonic generation [10] (SHG) still is one of the most important methods to achieve intense coherent blue light with a good optical beam quality [11]. From symmetry aspects, SHG is only possible in crystals belonging to one of the 20 point groups which lack a center of symmetry [12]. Furthermore, to meet phase matching aspects in homogeneous crystals, refractive indices for the fundamental and the generated harmonic wavelength must be of identical size [13,14], optical birefringence is necessary. Here we discuss the properties of benzophenone (C 13 H 10 O), an organic crystal, which excellently meets the described conditions. Crystal Properties Crystals of benzophenone can be grown from the melt (melting point approximately 48°C) [15]. The crystal structure is orthorhombic with point symmetry 222. This forces the material to be optically biaxial with the axes of the indicatrix fixed to the three mutually perpendicular symmetry axes of the crystal. Theoretical studies revealed - due to the conjugated bonds - a good second order nonlinear optical polarizability [16]. Optical Absorption For optical applications in general - and especially for SHG - the material considered must be transparent in the wavelength region of interest. Measurements of the optical absorption in benzophenone show a region of excellent transparency spanning from 390 to 1700 nm. Depending on light polarization - parallel or perpendicular to the crystallographic c-axis - two typical spectra are found. The experimental results are sketched in Fig. 1.