Aptamers, including naturally existing riboswitches, are single-stranded nucleic acids (DNA or RNA) and peptides molecules, which show the high affinity, selectivity, specificity, and versatility. Aptamers are selected by using the systematic evolution of ligands by exponential enrichment (SELEX) method against a particular target molecule. Aptamers can target the variety of molecules like proteins, carbohydrates, bacteria, viruses, toxins, and cancerous cells and can discriminate between closely related ligands. Aptamers used various strategies to sense the target binding like electrochemical, optical, mass-sensitive analytical technique etc. Thus, aptamers magnetizes the immense interest in the field of pathogen diagnostic and detection. In this chapter, we describe how the different strategies have updated for the isolation and selection of more efficient, novel and specific aptamers. In the last, we also provide insights about the potential aptamers used for the detection several biothreat agents like bacteria and parasites, and viruses.