This book provides a comprehensive and easy-to-understand description of the complete Fortran 90 programming language as defined by the ISO. Fortran is one of the most widely used programming languages in science and engineering with around 15% of code worldwide being written in Fortran. Fortran 90 Language Guide is designed to appeal directly to programmers, demonstrating the many features of the language which are new to this version - including those which support object-oriented programming and high-performance computing. Fortran 90 allows programmers to write code faster, make it more legible, and avoid many bugs. All Fortran users should ensure they keep a copy of this book close to hand.