  Unmeasureability of a quantum state has important consequences in practical
implementation of quantum computers. Like copying, deleting of an unknown state
from among several copies is prohibited. This is called no-deletion prinicple.
Here, we present a no deleting principle for qudits. We obtain a bound on
$N$-to-$M$ deleting and show that the quality of deletion drops exponentially
with the number of copies to be deleted. In addition, we investigate
conditional, state-dependent and approximate quantum deleting of unknown
states. We prove that unitarity does not allow us to delete copies from an
alphabet of two non-orthogonal states exactly. Further, we show that
no-deleting principle is consistent with no-signalling.
