  This chapter summarizes the properties of near-threshold molecular states
that can be formed from ultracold atoms. The properties of the states for a
single potential are characterized for different partial waves using the
concepts of quantum defect theory for a van der Waals potential. Examples are
given for various isotopes of the Yb_2 molecule. Magnetically tunable Feshbach
resonances or photoassociation resonances are characterized by the multichannel
version of the theory. Examples of the latter are given for the KRb molecule.
