The Z-scan technique is a very simple, straightforward, and highly sensitive technique that can be used to measure the refractive and absorptive nonlinearities of a variety of materials. In this technique, the transmittance of a sample is measured at the far field while the sample is scanned along the propagation path of a focused Gaussian beam. The analysis of the Z-scan profiles is used to extract all the nonlinear photophysical parameters. This technique is widely used to find novel nonlinear optical materials for photonic applications. Nowadays, this technique has also shown its practicality as a diagnostic tool for various diseases. Due to its low cost, high sensitivity, and widespread applicability, the Z-scan technique has seen acceptance in a variety of therapeutic fields for the diagnosis of cardiovascular diseases, cancer, neurodegenerative diseases, liver and renal disorders, bacterial and viral infections, etc. In addition, the Z-scan technique is utilized to determine the relative effectiveness of various treatments. This chapter presents a concise introduction to the fundamental concepts of the Z-scan technique, encompassing both their theoretical and experimental aspects. Additionally, it discusses the recent potential applications in disease diagnosis.