The passive locomotion of a body placed in the flow of periodically-generated vortices is studied. This work is motivated by recent experimental evidence that live and dead trout exploit the vortices in the wake of an oscillating cylinder to swim upstream. We consider a simple model of a rigid body interacting dynamically with point vortices introduced periodically into the flow to emulate the shedding of vortices from an external source. We show the existence of periodic solutions where the body `swims' passively against the flow by extracting energy from the ambient vortices. We also find solutions where the body holds station in the incoming wake. However, for bodies of elongated geometries, rotational instabilities may hinder their motion. We propose active feedback control strategies to overcome these instabilities. (This is joint work with my graduate student Babak G. Oskouei)