This book is a compilation of most works published during 2000-2016 in the area of dry powder inhalers, especially related to the issues with formulation and device design. The dry powder inhalers aim to deliver the medication to the respiratory airways. It is suitable for postgraduate students and researchers who work in the areas of dry powder inhalers. It provides some background knowledge in size characterization, flow properties, forces, and interaction of air and particles. It also ends with in vitro quality control of dry powder inhalers. This book was finished in Hawaii where the author spent his time during April 2016. Thank you to Prince of Songkla University for the financial support. The author had to work very hard for one month without a holiday. Thank you to Professor Aran Pattanothai for overseeing this work to make sure that the researchers were on schedule all the time. Thanks to the authors staff at the Graduate School, Prince of Songkla University who had to work hard and be patient when the author was not in the office. Thanks to the authors wife and son who had to wait for him at home in Thailand. Thanks to Dr Tan Suwandecha and Dr Janwit Dechraksa for their great efforts in copy editing and compiling all the references. The author also thanks Dr Somchai Sawatdee and Dr Dhamodharan Bakkiyaraj for their comments, and Dr Padmavathi Alwar Ramanujam for her proofreading. The greatest help was from Professor Alan Coombes, Professor Pornanong Aramwit, and Dr Brian Hodgeson for their criticisms and comments to make this book readable.