Assessment for Learning (AfL) may be conceptualized as minute-to-minute, day-by-day interactions between learners and teachers with the improvement of learning as the principal focus. This paper traces the development of an AfL measurement instrument (scale) that can be used for research purposes prior to, during and following professional development in the area. Rasch measurement procedures were applied to data drawn from a convenience sample of 594 teachers from 44 elementary schools in Ireland to create a scale consisting of 20 items distributed across four key AfL assessment strategies: learning intentions and success criteria, questioning and classroom discussion, feedback, and peer-and self-assessment. This scale, the Assessment for Learning Measurement instrument (AfLMi), has good psychometric properties and is interpretable in a way that makes it potentially useful during system wide improvement initiatives focused on AfL.