The effects of injection locking a laser diode which is also subject to varying levels of optical feedback from an external reflector are investigated. It is found that the locking range of a laser diode with optical feedback (LDWOF) is reduced relative to that of a solitary laser diode depending on both the increased photon lifetime and the amount of optical feedback. When the LDWOF is locked in frequency to the external injection source then the coherence collapse, and other low optical feedback regimes of operation, are suppressed. This result can be applied to produce a self-locked or master/slave-locked diode laser system that is insensitive to unwanted feedback typically encountered in practical communication systems. The effect of injection locking a frequency modulated external cavity diode laser (FM ECDL) system by another laser is to inhibit the modulation induced coherence collapse that is characteristic of such FM systems. Due to this, larger bandwidths are attainable than for free running FM ECDLs. This also suggests future device and system designs for obtaining very broad optical bandwidth FM operation.