This textbook on Reliability Theory focusses on Applications in Preventive Maintenance (PM). All models are presented in connection with the relevant statistical material. Short and simply written the book is almost self-contained. The reader needs not more than basic knowledge of calculus, probability and statistics. Each chapter is concluded by a series of exercices with detailed solutions. Numerical solutions are elaborated with Mathematica software. Novel topics are discussed, like PM with learning, choice of the best time-scale for PM, handling multidimensional state description, dealing with uncertainty in data. The book is meant for graduate students, researchers and engineers specializing in Quality Control, Logistics, Reliability and Maintenance Engineering.