This thesis explores the polarisation properties and lasing characteristics of externally driven vertical cavity surface emitting lasers (VCSELs). Polarisation properties, including polarisation state selection and polarisation switching of VCSELs, were investigated experimentally using various schemes of optical injection and optical feedback. Optical injection and optical feedback are known to induce a diverse range of effects in VCSELs, which can be advantageous to the operating characteristics of such devices. New approaches, namely circularly polarised optical injection and circularly polarised optical feedback, have been used experimentally for the first time. Using circularly polarised externally optical injection, the output polarisation of electrically pumped VCSELs can be strongly influenced. The linear polarisation of VCSELs emission can become circularly polarised for bias currents below or near the threshold current of the stand-alone VCSELs. In addition, using a new circularly polarised optical feedback scheme, the VCSELs emission was made to exhibit a degree of circular polarisation. The degree of circular polarisation depends on the feedback power ratio and the VCSEL bias current. The role ofthe suppressed mode in the polarisation switching characteristics of VCSELs was investigated using different forms of linearly polarised optical injection. The minimum injection power for polarisation switching to occur has been found to decrease dramatically with increasing VCSELs bias current. Polarisation switching in multimode VCSELs was investigated using optical injection. Irreversible polarisation switching in two-mode operation VCSELs was observed using parallel/orthogonal optical injection. Furthermore, polarisation bistability and ultra-wide hysteresis were obtained for two-mode operation regime with orthogonal optical injection. In addition, lasing threshold characteristics of directly modulated VCSELs was demonstrated experimentally. Dynamical hysteresis, depending on thermal effects, in the lasing threshold turn-on and turn-off were observed and compared to theoretical predictions.