The article deals with theoretical and methodological problems related to research on friendship during adolescence. It presents methods used in psychological research for determining the specificity and essence of friendship and its applied measurement A review of such completed studies includes not only the results and views showing the developmental advantages of friendship but also indicates the negative influence of a close friend (the so-called 'friend effect'). The reason for such resultant ambiguity may be the lack of a uniform and comprehensive theory of friendship and/or the specificity of applied methods (questionnaires) and research procedures (quantitative research with the use of large samples). The proposed concept is one developed by Aelred of Rievaulx, which introduces different kinds of friendship and connect its essence to a transcendental dimension. The paper also includes the specificity of qualitative methods and some research results from their application. Pursuant to her own research experience, the author has formulated the rules of research on friendship with the use of qualitative methods.