Over the last few decades, major developments in sensors have appeared in the form of many technologically innovative devices. Sensors play very important roles in the detection of heavy metals, pesticides, toxic gases, organic molecules, infectious organisms, etc., for the betterment of the environment and health. The newer sensors have several advantages, such as the requirement for very low sample concentrations, cost-effectiveness, quick response, and point to care capability. Based on to the unique properties in the electrical, optical, and magnetic fields of nanomaterials, nanosensors have become important devices in modern technology. Due to increasing demand for low-cost sensor devices, the nanosensor has experienced significant growth as an area of nanotechnology. With this in mind, the current chapter focuses on different aspects of designing methods and fabrication processes to build nanosensors. In the initial part of the chapter, we discuss the introduction of sensors and their classifications as electrochemical nanosensors, bionanosensors, or chemical nanosensors. In the next part of the chapter, we place our focus on a detailed discussion of the designing and fabrication of nanosensors. Finally, in the last part of the chapter, we state a brief analysis of the entire discussion, along with future perspectives.