This book is about how to develop and evaluate multimodal systems which are usable by, or fit, people. The main objective is to answer the practical question of what system developers need to know and be able to do in order to develop usable new multimodal applications. The need to know is addressed in the first part of the book, in which the role of development and evaluation for multimodal usability in the software engineering life-cycle is described, and 9 key multimodal usability parameters are presented as well as theory of modalities and multimodality. The need to be able to do is addressed in the second part of the book. The distinction between interleaved and iteratively performed (i) development for multimodal usability and (ii) evaluation for usability is discussed.