The purpose of this article is to clarify the concept of the critical friend. It is practically used in self-study research, which is a kind of qualitative study. Self-study as a systematic inquiry helps teachers and teacher candidates to explore and analyze their practices. Self-study as a qualitative research methodology is largely recognized in western countries yet considered relatively new in Turkey. In this article, the history of self-study research methodology is briefly introduced and how it differs from other qualitative research methods is explained. Then, the concept and importance of critical friendship, which is one of the key characteristics of self-study methodology because of its collaborative nature, is explicated. Critical friends are reliable colleagues who systematically provide data to the researcher and at the same time are those who help increase the validity and reliability of the research. Since it is hard for the researchers to reframe their experiences and verify assumptions objectively, critical friends provide pivotal support by asking challenging questions and presenting constructive criticism. For this reason, choosing a critical friend considerably affects the whole research. This article attempts to specify attributes and qualities of a critical friend and answer the question "What kind of a critical friend is needed in a self-study?"