Thiosemicarbazone compounds are established as selective and sensitive spectrophotometric reagent for cobalt.This book describes literature survey and synthesis of some important thiosemicarbazones. HBBrAT and HBBrPT were developed as new reagent for spectrophotometric determination of micro quantities of cobalt.This extractive technique is found to be very simple, rapid and accurate for detection and determination of cobalt.The details of selection of pH, wavelength and solvent for the present system is reported. Validity of Beer"s law, molar extinction coefficient, Sandell"s sensitivity, Effect of foreign ion, etc. were investigated.Job"s method of continuous variation, the mole ratio method and the slope ratio methods were studied. The stability Constant and free energy of the cobalt complexes were calculated. This book is very much useful for post graduate and research scholars to determine cobalt spectrophotometrically.