In this paper, illusion is examined as a prerequisite and necessary medium for the analysand's finding the genuine subjective reality of his own in the psycho-analytic interaction. Two kinds of illusion are discussed. The first of them, transference illusion, is well-known, as is its understanding as a simultaneous existence of experiences stemming from different levels of reality. At its side, the author introduces the concept of developmental illusion, as an essential constituent of the analytic process. As contrasted to transference illusion, the wishes inherent in a developmental illusion have never become meaningfully represented in the analysand's mind. These interrupted developmental needs attaining shape and meaning, and thus the possibility to develop further in the analytic relationship, is dependent on the analyst's ability and ways to receive and meet the analysand's activated developmental illusion. The rôle of the analyst's ways to reach and convey his understanding is considered decisive in this process.