Nanosensors are cutting-edge analytical systems that integrate molecular or atomic-level sensing elements with physical transducers. These sensors utilize nanostructured components, such as carbon nanotubes, graphene, nanowires, or nanoparticles, due to their high surface area-to-volume ratio and superior sensitivity. They can convert binding occurrences or chemistry on the nanostructured surface into detectable forms, including electrical, optical, thermal, or electrochemical signals. Their high selectivity and sensitivity are based on the unique chemical and physical characteristics of nanomaterials and advanced nanofabrication techniques. Key performance parameters include the limit of detection, selectivity, linearity/range, and response time. Nanosensors have diverse applications, ranging from disease diagnosis and detection, food examination, and air and water purification, to security and defense. They offer cost-effective production, minimal sample volumes, and the potential for integration into portable systems. Notwithstanding their immense potential, challenges related to integration onto device substrates, reproducibility, stability, non-specific binding issues, and sensor calibration exist. Nevertheless, advancements in nanotechnology and nanoengineering are expected to rapidly enhance the accuracy and speed of nanosensors, paving the way for their widespread adoption across various applications. This chapter will explore the fundamentals of nanosensors, providing an overview of their fabrication methods, the different types of nanosensors, and their wide range of applications.