This feature article reviews the emerging applications of activatable nanomaterials in the bio- and medical fields, namely, molecular imaging, theragnostics, cancer therapy, tissue engineering, and biosensors. Nanomaterials, with an ideal size to interact with biological systems replete with nanoscopic biomolecules, are now finding applications in previously inconceivable tasks of affecting biological systems at the sub-cellular level, revolutionizing the technological developments of the bio- and medical fields. This feature article places special emphasis on the nanomaterial designs, the activation mechanism of the prepared materials and the nanoscopically localized effect of the resulting nanomaterial-originated events, which are designed to manipulate the behavior of biological systems or obtain critical biological information.