Biosensors have become an important research area, owing to their potential uses across a wide range of fields, varying from environmental monitoring, human health management, and food and water safety. The most promising options for creating sensitive, quick, and affordable biosensors are various carbon allotropes, which have been thoroughly investigated in works relevant to biosensors. The use of carbon nanotubes (CNTs) for various biosensors is covered in this chapter. The excellent physical and electrical characteristics of carbon nanomaterials contribute to their popularity in the development of biosensors. High sensitivity and specificity biosensors can be made by combining these materials with sensing components. In addition, CNT-based biosensors possess the advantages of online monitoring, low cost, and quick analysis in complex systems. They are also anticipated to be integrated and downsized to cut down on sample size and power consumption. All these merits promote the prompt development of biosensors in many applications.