During the last few years, new discoveries in the RNA field have opened up a wealth of opportunities to specifically target mRNA for functional analysis and target validation. Contained in this volume of Methods in Molecular Biology are methods useful for the design and application of ribozymes, DNAzymes, and small interfering RNAs (siRNAs). In addition, a number of illustrations aiming to facilitate the und- standing and application of each method is included. Like any new field, ribozymes and siRNA research is rapidly evolving, and so too are the methods used to study, select, express, and control their structure and in vivo biological activities. Ribozymes and siRNA Protocols, Second Edition focuses on the latest technical advances in interfering with gene expression. It contains a short int- duction followed by general and specific protocols for using hammerhead ribozymes and derivates, DNAzymes, hairpin ribozymes, group I intron ribozymes, RNase P ribozymes, and siRNAs. Also in this volume are general methods for RNA structure analysis, delivery of oligonucleotides, and gene therapy protocols using ribozymes.