  Superfluid 3He-A displays the largest variety in vortex structure among the
presently known coherent quantum systems. The experimentally verified
information comes mostly from NMR measurements on the rotating fluid, from
which the order-parameter texture can often be worked out. The various vortex
structures differ in the topology of their order-parameter field, in energy,
critical velocity, and in their response to temporal variations in the
externally applied flow. They require different experimental conditions for
their creation. When the flow is applied in the superfluid state, the structure
with the lowest critical velocity is formed. In 3He-A this leads to the various
forms of continuous (or singularity-free) vorticity. Which particular structure
is created depends on the externally applied conditions and on the global
order-parameter texture.
