This workshop aims at initiating a dialog between theoreticians and practitioners, aiming to bridge the theory-practice gap in this area. The workshop will be built along three main question: FROM THEORY TO PRACTICE: Which abstract theoretical characterizations / properties / statements about clustering algorithms exist that can be helpful for practitioners and should be adopted in practice? FROM PRACTICE TO THEORY: What concrete questions would practitioners like to see addressed by theoreticians? Can we identify de-facto practices in clustering in need of theoretical grounding? Which obscure (but seemingly needed or useful) practices are in need of rationalization? FROM ART TO SCIENCE: In contrast to supervised learning, where there is general consensus on how to assess the quality of an algorithm, the frameworks for analyzing clustering are only beginning to be developed and clustering is still largely an art. How can we progress towards a deeper understanding of the space of clustering problems and objectives, including the introduction of falsifiable hypotheses and properly designed experimentation? How could one set up a clustering challenge to compare different clustering algorithms? What could be scientific standards to evaluate a clustering algorithm in a paper? The workshop will also serve as a follow up meeting to the NIPS 2005 “Theoretical Foundations of clustering” workshop, a venue for the different research groups working on these issues to take stock, exchange view points and discuss the next challenges in this ambitious quest for theoretical foundations of clustering.