The GTC (Gran Telescopio Canarias) employs an Acquisition and Slow Guiding Sensor based on a Marconi CCD47-20. This chip is also the core sensor for the telescope Segment Figure Sensor. A third sensor, based on a Marconi CCD39-01 implements Fast Guiding and is also used for Wavefront Sensing under continuous rotation. 1 Hz full-frame readouts and 10 Hz window readouts are performed onto the slow guiding sensor, whereas 200 Hz full-frame readouts are required for the fast guiding sensor in binning modes. GTC sensors are low-profile, lightweight, individual CCD heads with integral low-noise preamplifier stages and Peltier cooling electronics. These CCD heads are liquid-cooled and include forced, dry air circulation. Besides, high noise inmunity, proper shielding and grounding and impedance matching of the cabled links have been major design concerns. Sensor readout is performed by a Magellan CCD Controller (Greg Burley - OCIW), which has undergone modification at GTC in order to include additional analog input channels and to provide support for off-board, cable-linked sensors. The sensor's built-in electronics are also derived from the original Magellan design. We describe the detailed design of the GTC sensors and the modifications carried out in the Magellan controller.