The use of Web 2.0 communication practices in assessment may offer a new approach to assessment "as" learning, a goal for assessment that is more highly valued than assessment "of" and even "for" learning in the literature. This paper describes a case study of a wiki-writing assignment that aimed to achieve this goal. The stakes were high, in that the assignment was worth 100% of the mark and the assignment was set for a group of tertiary teachers who were enrolled in a unit of study about student assessment as part of a Graduate Certificate in Tertiary Education. This paper analyses how the wiki-writing assignment shaped assessment "as" learning in four ways, through: finding a language for talking about assessment; collaboration (interaction); dialogue (expression); and inquiry learning (cognition). (Contains 1 table.)