This paper reports on velocity, pressure, and correlation measurements in the turbulent wake and the adjacent unsteady potential flow of a circular cylinder. Particular attention is given to the effects that attend oscillation of the cylinder in the plane of the lift force. The study was undertaken for four reasons. First was the need to generalize measurement results on the lift force of an oscillating cylinder placed in a flow [1]. Second was the desire to clarify the fluidelastic nature of such forces. Third was the wish to compare gross features of real and computer generated flows. And fourth was the presumed possibility to create unsteady flow of controlled characteristics by using vibrating cylinders. The data presented here pertain to the velocity field close to the cylinder. Actually the records for locations further downstream suggest that prospects for control of flow characteristics using vibrating bluff cylinders do not look promising beyond, say, 15 dia. Measurements made close to the cylinder permit more definite conclusions. The fluidelastic nature of the flow associated with vortex-induced vibrations of cylinders has been demonstrated conclusively. Various existing notions about this question can now be segregated. Reynolds numbers of up to 105 were attained; this is sufficiently high to give results in the range of practical application. Revealing records about the flow field structure were acquired. They permit many conclusions by mere visual inspection because of the special attention given to the unsteady or potential flow region.