Although collaboration among service providers has been a hallmark of special education almost since its inception, co-teaching, the sharing of instruction by a general education teacher and a special education teacher or another specialist in a general education class that includes students with disabilities, is a relatively recent application. As a result of recent federal legislation and related policy changes, co-teaching has evolved rapidly as a strategy for ensuring that these students have access to the same curriculum as other students while still receiving the specialized instruction to which they are entitled.Despite considerable enthusiasm expressed by those who write about co-teaching and those who implement it, co-teaching illustrates the complexity of conceptualizing and studying collaboration in special education. Most inquiry on co-teaching has emphasized co-teachers' roles and relationships or program logistics rather than demonstrating its impact on student achievement and other key outcomes, and far more literature exists describing coteaching and offering advice about it than carefully studying it.Contributing to the admittedly equivocal evidence base for co-teaching are factors such as the still emerging understanding of this special education service delivery vehicle, inconsistencies in definitions and implementation, lack of professional preparation, Correspondence should be sent to Marilyn Friend,