This book constitutes the thoroughly refereed post-proceedings of the 9th International Workshop on Declarative Agent Languages and Technologies, DALT 2011, held in Taipei, Taiwan, in May 2011. The volume contains 6 revised selected presented at DALT 2011, 7 best papers from the DALT series over the years, explaining how the research developed and how it influenced and impacted the community, the state-of-the-art and subsequent work, and two invited papers from the DALT Spring School, which took place in April 2011.