Over the last few years, the use of phosphodiesterase type 5 (PDE-5) inhibitors increased exponentially in individuals of varying ages. Three highly selective and potent PDE-5 inhibitors mainly used are: sildenafil, tadalafil and vardenafil. These drugs are used for the treatment of erectile dysfunction and also for recreational purposes. Moreover, It has been well known about the effects of PDE-5 inhibitors on diabetes mellitus, hypertension, cardiovascular diseases, depression, multiple sclerosis, spinal cord injury, chronic renal insufficiency, renal transplants, liver disorders and other conditions. This book, provides new pieces of knowledge about the effects of these drugs on physical performance and on endocrine system. This knowledge could help the fitness coaches and all the people that are involved in sport. However it could be of a great significance also for people who use PDE-5 for other than physical performance reasons. At this moment we recommend a great caution in transferring our results in the areas of sport and doping.