The vertical cavity surface emitting laser (VCSEL) is a relatively new se- conductor laser device, especially applicable to ?ber-optic networks in the 21st century. About 25 years have passed since its invention, and devices for Gigabit Ethernet are now being mass-produced. It is expected that VCSELs will open up a new era of very-high-speed ?ber-optic networks and a wide range of application areas. VCSEL have many advantages, such as: (i) monolithic and high-yield f- rication;(ii)extremelylowthresholdandsmallpowerconsumption;(iii)hi- speedmodulationcapabilityatlowdrivingcurrentlevels;(iv)easypackaging, and so on. The emission spectra have been extended from 850nm toward the visible and infrared regions. We can expect reasonably high-power output, enough for most applications, and extremely high powers in some engineered arrayed devices. This book project was initiated to provide important information on VCSELs, edited by Prof. Herbert Li. Very regrettably, while editing this book he passed away. After some time, the present editor succeeded him. Due to this reason the publication of this book was delayed, and the editor would like to thank the contributors for their patience. This book includes the basic concepts, device technology, and application areas of VCSELs, and can be read not only by scientists and engineers in the ?eld, but also by graduate course students who wish to start their research or to get an introduction to the vast knowledge on semiconductor lasers.