A controller for use with a guidewire, such as a vascular guidewire, provides a mechanism for gripping and applying a torque to the guidewire without the need to thread the guidewire axially through the controller and at a location close to a point of access of the guidewire. In one embodiment, the controller includes a side-access, multi-part assembly including a collett or other gripping element that applies a uniform radially inward force on the guidewire. In another embodiment, for use with guidewires having active electrically controllable elements, the controller integrally or removably incorporates a switch or other mechanism to initiate an energized state. The controller thereby permits, among other advantages, ergonomic, single-handed control of an electronically steerable guidewire, including axially displacing, torquing and steering the guidewire.