Polycrystalline diamond possesses high hardness and wear resistance, among other superior properties, such as low coefficient of friction, high thermal conductivity, high corrosion resistance and low electrical conductivity. Some of these properties favor the application of polycrystalline diamond in oil & gas industry. PDC cutters are the primary and key components in the PDC drill bits to cut various formations. However, drilling very hard and highly abrasive formations poses a big challenge for today’s PDC drill bits. The weakness in the current technology is due to the unavoidable use of metallic catalysts to bond the diamond grains that comprise the PDC cutters in traditional high-pressure and high-temperature (HPHT) manufacturing. Development of catalyst-free PDC cutters would be a game changing technology for drill bits to potentially realize the goal of “One Run to Total Depth” in drilling technology. This chapter will cover the development and applications of both catalyst-synthesized and catalyst-free polycrystalline diamonds in oil and gas industry including latest breakthrough on ultra-HPHT manufacturing technology to make the hardest diamond on earth for drilling and completion. Besides the application of polycrystalline diamond as PDC cutters, this chapter will also cover its applications in bearing or drilling system.