The method of assessment in "Integrated Studies" which has begun in April 2002, has been entrusted to each school. Under the current situation teachers are concerned about the technique and what to evaluate but not enough discussion has taken place to clarify the theoretical framework of assessment in "Integrated Studies". In Japan, portfolio assessment is considered to be the most suitable method of assessment in "Integrated Studies". However, it is essential to first know how to interpret the evaluative information of portfolios if they are to be used in "Integrated Studies". This paper will examine the progress of the development of portfolio assessment and its characteristics to establish a suitable framework to interpret the evaluative information that is collected in portfolios. As a result, it has become clear that both "criterion-referenced assessment" and "self-evaluated assessment" are necessary if portfolios are to be used as an assessment for "Integrated Studies". In other words, it is necessary to have two types of criteria as an effective method of assessment. Firstly "criterion-referenced assessment" which is criteria established by the teacher and is "outside" the learners, and secondly "self-evaluation" which assesses the criteria set by learners themselves. In establishing these two types of criteria, it is necessary that the conditions to be decided and modified should be done "in conference" between the teacher and the individual student. This method of assessment enables the teacher and student to reach an agreement on the criteria used. The students can comprehend the evaluation process and become more involved in their learning. This enhances student involvement by allowing them to be a direct "stake holder" in their learning. Furthermore, there are specific "domains" set and given "standards" which are necessary to interpret the criteria. The aim of "Integrated Studies" is to increase the students' ability and skills to learn on their own and think about the way they can live their lives. Therefore, the method of assessment for "Integrated Studies" should not be based on "domain-referenced assessment" which analyzes criteria strictly. The best method is one using a "standard-referenced assessment" which is made from examples, descriptions and records of the students' work, which has been established by the teacher and student.