This thesis presents new spectroscopic observations of the donor star in several High Mass X-ray Binary systems. The various types of X-ray Binary systems that exist are outlined and the methods and limitations of mass determination of the stellar components are discussed. The masses of the compact objects are determined for the systems SMC X-1 and LMC X-3. In both cases the heating of the donor star by the X-ray source is taken into account and its importance demonstrated. In the case of SMC X-1, we determine the radial velocity semi-amplitude of Sk 160 to be 24.7 ± 1.9 km s-1. Assuming Sk 160 fills its Roche-lobe we obtain upper limits for the mass of the neutron star as Mx = 1.36 ± 0.10, and by assuming that the system is seen edge on we obtain lower limits for the masses of the neutron star as Mx = 1.05 ± 0.09 M⊙. However, based on the model predictions of Timmes et al. (1996), it is concluded that the mass lies in the upper end of the heating corrected mass determinations. In the case of LMC X-3 the spectral type of the donor star is observed to change with phase due to irradiation by the X-ray source. We find the spectral type is likely to be B5V, and only appears as B3V when viewing the heated side of the donor. Combining our measurements with those previously published, and taking into account the effects of X-ray irradiation, results in a value for the donor star radial velocity semi-amplitude of Ko = 256.7±4.9 km s-1. Assuming an inclination of 50° ≤ i ≤ 70° we find the mass of the black hole lies in the range 9.5 ± 1 M⊙ ≤ Mx ≤ 13.2 ± 1 M⊙. The idea of using these mass determination techniques as a tool for detecting potential black hole/Be star XRB systems is presented and the radial velocities for seven such systems are determined. Of these, one system, LS 4356 shows a radial velocity variation large enough (i.e. ~ 95 km s-1) to indicate the presence of a black hole. If the compact companion is confirmed as a black hole, it will make LS 4356 the first known black hole/Be XRB. Finally the emission lines observed in the spectra of two Be XRBs, X Per and A0535+26 are analysed and compared to the previous studies of Clark et al. (2001) and Grundstrom et al. (2007). The Hα emission line profiles are discussed in terms of their evolution with time and the changing state of the circumstellar disc, thereby confirming and adding to the long term data set. The emission line profiles of the other spectral lines, namely Hβ, H-γ, He I 5876, He I 6678 and He I 7065, are presented and compared with that of Hα.