Urodynamics (UDS) is an interactive diagnostic study of lower urinary tract function. It is composed of several tests that can be used to obtain functional information about urine storage and expulsion. Its main goal is to reproduce the patients' symptoms and determine their cause. The present article is a review of the physiological concepts behind UDS, and explains the various normal and abnormal forces and parameters that are measured and used during the tests to assist the physician in making a diagnosis. It outlines the importance and methods of the calibration of UDS equipment to optimise diagnostic accuracy and reliability, which would have a crucial impact over the treatment's decision, and consequently the patient's outcome.