The terahertz region offers a multitude of possibilities for new diagnostic techniques in biology. Recent advancements in terahertz sources and detectors have allowed such systems to be employed in investigations of biological tissues. The main interaction with such media is evident in the water content, which can be detrimental in certain instances such as deep tissue diagnostics and also be beneficial in others such as the detection of certain types of cancers. The field has developed to deal with such challenges, and the improvement in overall signal to noise of especially pulsed diagnostic methods has led to the achievement of penetration depths on the order of millimeters, which has opened up a variety of new investigations. These investigations are described with special emphasis on diagnostics on hard tissues such as teeth.