  Einstein-Podolsky-Rosen (EPR) steering is a form of bipartite quantum
correlation that is intermediate between entanglement and Bell nonlocality. It
allows for entanglement certification when the measurements performed by one of
the parties are not characterised (or are untrusted) and has applications in
quantum key distribution. Despite its foundational and applied importance, EPR
steering lacks a quantitative assessment. Here we propose a way of quantifying
this phenomenon and use it to study the steerability of several quantum states.
In particular we show that every pure entangled state is maximally steerable,
the projector onto the anti-symmetric subspace is maximally steerable for all
dimensions, we provide a new example of one-way steering, and give strong
support that states with positive-partial-transposition are not steerable.
