The theory of generalised measurements is used to examine the problem of
discriminating unambiguously between non-orthogonal pure quantum states.
Measurements of this type never give erroneous results, although, in general,
there will be a non-zero probability of a result being inconclusive. It is
shown that only linearly-independent states can be unambiguously discriminated.
In addition to examining the general properties of such measurements, we
discuss their application to entanglement concentration
