This book, the first volume in the Transactions on Aspect-Oriented Software Development series, presents nine revised papers that have been through a careful peer reviewing process by the journal's Editorial Board. The papers cover a wide range of topics from software design to implementation of aspect-oriented languages. The first four articles address various issues of aspect-oriented modeling at the design level; the following four articles discuss various programming language issues.  The final article in this volume describes a workbench for implementing aspect-oriented languages, so that easy experimentation with new language features and implementation techniques are possible.