This ebook is a collection of cutting edge articles from the 2009 Workshop on Microcavities and their Applications (WOMA 2009). It gives readers an overview of state-of-the-art opto electronic research on nano and micro cavities presented by leading experts in the field. Two major themes covered in this ebook are (i) wave- and ray-chaos in open microcavity systems and (ii) opto-electronic applications in nano and micro cavities. The book concludes with a description of recent advances in 2D whispering-gallery-mode microresonators. This ebook should be a very useful reference for any serious opto-electronics researcher.