This book describes successful attempts made in research laboratory for developing the extrinsic fiber optic sensors for different industrial applications. The description in the book is mainly written keeping the focus on research and industry community. Different configurations of retro reflective type fiber optic sensor sensors are evolved while developing the sensors for identified application. Generalized mathematical model helps in simulating different sensor configuration in terms of different performance parameters. A design help tool FOS EXPLORE gives the optimized configuration in terms of geometrical and physical parameters. This helps in designing the sensor without any tedious trial and errors. Selection of components in terms of type of source, detector and reflector is a key issue. Detailed description of selection procedure is given in this book. Various laboratory sensor prototypes are described considering fabrication and testing aspects of each.