The development of critical thinking as an essential skill in 21st-century learning is uncontested within educational and professional settings. The degree to which it is operationally defined, taught and assessed, however, is not well documented. This complicates efforts to develop critical thinking in learners, as well as devise intervention techniques and assessment tools. The ACER critical thinking skill development framework has been developed to address the challenges associated with teaching and assessing critical thinking. While there are many definitions of the skill, which are outlined in the first part of this document, few provide a means to operationalise critical thinking in the classroom. This framework outlines critical thinking processes along prescribed strands and aspects informed by a sound evidentiary basis. The aspects contained within the framework are designed to provide foci for teaching and the basis of assessment. Aligned with the nature of the classroom, the proposed framework characterises critical thinking as cognitive processes that are ultimately goal directed and purpose driven. Whether that purpose is to solve a problem, support a theory or statement, conduct an experiment, formulate an argument, present an interpretation, undertake a critique, better understand a topic or decide on a course of action, the skills presented assume that critical thinking is not simply reflective thought; it is also applied and generative. As a teaching and assessment resource, the ACER critical thinking framework seeks to describe critical thinking as a generally applicable set of skills that can be operationalised in classroom practice. The skill can be described and understood in a generalised way that can be applied across disciplines, with this framework providing a consistent terminology in which to do so. The aspects can be used to write or map assessment items, or the aspects can be integrated into lesson plans. The skill needs to be embedded within the methodologies, conventions and ‘ways of knowing’ of each of the disciplines to give their application context, to ensure they are relevant, and that they can be sustainably integrated.