The main goal of this book is to introduce the required coefficient of friction (RCOF) as a new variable to characterize the normal and pathological gait; also, its might be used in the prediction of the real fall propensity of patients with stroke and Parkinson's Disease (PD). In the First Chapter the literature review of Tribology (the study of friction) and the RCOF during the gait are presented. The Second Chapter discusses the methodology to calculate the RCOF based on the ground reaction forces (GRF). This variable is calculated as the ratio between the shear of the horizontal GRF components (resultant of lateral and anterior posterior GRF) and the vertical GRF. With this methodology the studies presented in the Third, Fourth, Fifth and Sixth chapters were design. The Third and the Fourth chapters the studies about the effects of flooring in RCOF during normal and pathological gait were discussed. In the fifth and sixth chapters, studies about the instantaneous RCOF curve analysis in patients with Stroke and PD were presented respectively. These analyses represent the first attempt to explore the RCOF curve parameters during the gait analysis.