Langmuir probes are a long established tool for the investigation and char- acterization of plasmas. Diagnostic use of planar Langmuir probes in sta- tionary low temperature plasmas is a well understood and long established technique. When the plasma possesses a drift velocity greater than the ion sound speed the flow is considered to be supersonic. Under such conditions the theory for Langmuir probes is less than satisfactory. Where the flow is supersonic the Mach probe theory of Hudis and Lidsky [1] can be applied for a magnetized plasma. However in the case of an unmagnetized plasma there is no satisfactory theory. It has been observed that in orientating a planar Langmuir probe parallel to the direction of flow, the ion current due to the flow is eliminated. Under such conditions the behaviour of the plasma’s in- teraction with the probe bears close resemblance to the conditions seen in plasma immersion ion implantation (PIII). This thesis describes the adaptation’s made to PIII analytical model’s to take advantage of these similarities and its use to then describe the ion current of planar Langmuir probes in unmagnetized plasmas possessing a supersonic flow. In adapting a suitable analytical model for planar Langmuir probes under such conditions, extensive use of both 1D and 2D hybrid particle in cell (PIC) simulations have been made. Additionally the work required the development of a 2D hybrid PIC code where the probe is embedded within the grid. This allowed the investigation of the impact of edge effects on the analytical model’s performance. The theory for and structure of the 2D code is also presented as part of this work. Three different probe orientations are considered, firstly the parallel case, the other two concerns the near parallel cases of the probe surface orientated both into and away from the direction of flow. The model’s performance under these conditions is evaluated and discussed. Finally the use of this model in allowing a planar Langmuir probe to act as a Mach probe is also considered. In testing the success of the analytical model against experimental data, comparisons are made between the models results and those of the 2D hybrid PIC. The experimental results used for this work were for xenon plasma with a range of moderately supersonic velocities and a highly supersonic silver laser ablated plasma plume.