The rheological properties of solutions of high molecular weight (~2x10⁶) xanthan gum, a biopolymer commonly used as a drag reducer, viscosity enhancer in food applications, and as a drilling fluid in enhanced oil recovery are explored.  Polymer concentrations ranging from dilute to concentrated are studied using steady state, transient, and oscillatory rheology over a range of temperatures.  Under shear flow these solutions exhibit a Newtonian plateau (zero-shear rate region) at low shear rates followed by a region of shear thinning covering up to six decades of shear rate, depending on the polymer concentration.  The overlap and entanglement concentrations are determined to be 140 ppm and 1235 ppm, respectively from a plot of zero shear viscosity as a function of concentration.  A crossover in the dynamic moduli (G',G”) is observed for concentrations above the entanglement concentration while below the overlap concentration no crossover is observed.  Xanthan also undergoes a temperature-induced conformational change from the native double stranded helical structure to a solution of random coils.  The transition occurs in the range of 40 ^oC to 60 ^oC as evidenced by a sharp reduction in the solution viscosity.  This conformational change is time dependent and partially reversible.  The incomplete reversibility suggests imperfections in the structure when the random coils reorganize into double helices.  The effect of solvent ionic strength is also investigated.  Polymer solutions with 0.1M NaCl stabilize the native xanthan configuration, delaying the temperature-induced conformational change to much higher temperatures (~80 ^oC).