Nanodiamond (ND) particles play a vital role in diagnostic and therapeutic applications. The ND surface can be conjugated with various drugs/molecules for the purpose of interaction with various biological entities, such as cells, tissues, unicellular organisms, etc. The availability of methods to detect NDs and their biocompatibility with regards to various human and animal cells adds a significant advantage to their use. The prominence of ND surface functionalizations/conjugations and methods implemented to perceive their behavior after interaction with various human and animal cells are presented in this chapter. The antibacterial activity and the interactions of nanodiamond with microorganisms are discussed. This chapter also provides useful insights for future biophysical interactions of nanodiamonds in vivo.