The Galactic thick disk - Status

The status of the Galactic thick disk is reviewed. Consideration of the rec ent literature suggests that its vertical scale height and normalisation wi th respect to the thin disk remain uncertain to within a factor two, with v alues reported in the ranges 750-1500 pc, and 0.02-0.13, respectively. The bulk of the thick disk has kinematics (sigma(U), sigma(V), sigma(W)) = (65, 54, 38 km s(-1)), and lags the thin disk by some 40 km s(-1). differences of opinion exists as to whether kinematics change with distance from the Ga lactic plane. The bulk of the thick disk has [Fe/H] similar to -0.6, with l ittle or no evidence for a vertical gradient. The question of gradients is critical for an understanding of thick disk cosmogony and needs closer atte ntion. The reality of the so-called metal-weak thick disk (material having disklik e kinematics and [Fe/H] < -1.0) is also considered. The case for such mater ial seems to be steadily growing: in the range -1.6 < [Fe/H] < -1.0, recent estimates suggest rho(MWTD)/rho(Halo) similar to 0.1 - 0.3. While many workers regard the thick disk as a discrete entity, the caveat i s made that this is a sufficient condition, but not one necessarily require d by the observations. Best practice requires that both the discrete model and the alternative extended configuration be compared with observational d ata to examine the relative likelihood of their relevance. Recent theoretical advances are also discussed, together with the need for in situ measurements of the thick disk away from the Galactic plane.