  We give a description of the category of ordinary K3 surfaces over a finite
field in terms of linear algebra data over Z. This gives an analogue for K3
surfaces of Deligne's description of the category of ordinary abelian varieties
over a finite field, and refines earlier work by N.O. Nygaard and J.-D. Yu.
  Our main result is conditional on a conjecture on potential semi-stable
reduction of K3 surfaces over p-adic fields. We give unconditional versions for
K3 surfaces of large Picard rank and for K3 surfaces of small degree.
