Keywords: Euler equations, quantum fluids Abstract: Circulation is an often neglected conservation law in developing the mathematics of ideal fluids, meaning the classical 3D Euler equations and the quantum defocussing Gross-Pitaevskii equations. Recent Euler calculations demonstrated that the numerics must conserve circulation and when this is satisfied, it appears that circulation controls the growth of enstrophy in a manner consistent with a finite-time singularity of these equations. In a quantum fluid the circulation, that is defects in phase, is inherently conserved. These equations allow reconnection without dissipation and without singularities. Nonetheless, when compared with Navier-Stokes reconnection there are strong similarities, at least for a new Navier-Stokes initial condition which considers the role of circulation more carefully. Following reconnection in GP, waves form on vortex lines, vortex rings detach, and an inertial subrange develops, all in a manner that could explain experimental observations of the decay of vortex line length, a proxy for kinetic energy, despite the absence of viscosity.