The Wigner-Araki-Yanase (WAY) theorem delineates
circumstances under which a class of quantum measurements is ruled out.
Specifically, it states that any observable (given as a self adjoint operator)
not commuting with an additive conserved quantity of a quantum system and
measuring apparatus combined admits no repeatable measurements. I'll review the
content of this theorem and present some new work which generalises and
strengthens the existing results.
The observation that the WAY
constraint vanishes if the observable-to-be-measured is ``relativised'' (in a
suitable sense) points to interesting links with quantum reference frames and
superselection rules. I'll discuss some of these connections and raise some