A GelSight sensor is a tactile sensing device comprising a clear elastomeric pad covered with a reflective membrane, coupled with optics to measure the membrane’s deformations. When the pad is pressed against an object’s surface, the membrane changes shape in accord with mechanical and geometrical properties of the object. Since soft tissue is more compliant than hard tissue, one can de- tect an embedded lump by pressing the GelSight pad against the tissue surface and observing the hump that forms over the lump. We tested this system’s sensitivity by constructing phantoms of soft rubber with hard embedded lumps. The system is quite sensitive; for example it could detect a 2mm lump at a depth of 5mm. The sensor was more sensitive than previous tactile lump detectors. It was also better than human observers using their fingertips. Such a capability could help in tumor screening, and could augment the sensory information available in telemedicine or minimally invasive surgery.