Now and again, a topic emerges that reflects a critical mass of recent developments and that is particularly relevant to either digitization or digital preservation, our core areas of interest. Such topics often require more in-depth coverage than a single feature article and we devote an entire issue to it. This special issue on the certification of digital repositories includes three feature articles:

• Seamus Ross and Andrew McHugh present an introduction to audit and certification of digital repositories, as well as some relevant initiatives in which the Digital Curation Centre of the UK will engage
• Robin Dale discusses the development of certification methodology through the work of the RLG-NARA Digital Repository Certification Task Force, its recently released Audit Checklist for Certifying Digital Repositories, and the Center for Research Libraries Audit and Certification of Digital Archives project, funded by the Andrew W. Mellon Foundation
• Susanne Dobratz and Astrid Schoger provide an overview of two certification-related initiatives in Germany: the Deutsche Initiative für Netzwerkinformation (DINI) Certificate for Document and Publication Repositories and the Working Group on Trusted Repository Certification of the Network of Expertise in Long-term STOrage of Digital Resources (nestor)

You will see that a number of themes run through these articles including: the need to define, measure, and monitor the trustworthiness of digital repositories; the role of local, national, and international standards and best practice in evolving a digital repository certification program; the importance of incorporating both organizational and technological aspects into audit and certification approaches; and the significance and value of institutional self-assessment within the move towards full certification. We hope these articles will be useful to institutions in understanding the current state of developments pertaining to this core digital preservation topic and in determining the potential internal implications of these national and international developments. We also hope that compiling these reviews at a relatively early stage in the development of audit and certification processes will contribute to the cohesion of these initiatives in forging a unified, standardized, international approach.