Direct Measurements of V_tb
  The measurement of the single top-quark production cross section in hadron collisions at the Tevatron and the LHC can be used to determine the absolute value of the CKM matrix element V_tb without assuming the unitarity of the CKM matrix. By measuring the branching ratio of the decay t -> W + b one can relate V_tb to the matrix elements V_ts and V_td. The current experimental status and future prospects of these measurements are reviewed. 
Wolfgang Wagner
hep-ex