The main concern of this paper is to investigate the relation between self evaluation of the L1 and L2 linguistic competences by bilingual university students and the concepts of fossilization and attrition. The two terms refer to changes in language proficiency and imply a state of incompleteness of linguistic knowledge of both languages. Fossilization is identified as stagnation in attaining the target language proficiency whereas attrition is described as a loss of aspects of previously acquired linguistic knowledge. The former occurs at the level of active L2 development whereas the latter takes place at the state of post-active language acquisition. The paper constitutes an attempt to investigate the significance of the two concepts in the processes of bilingual development and maintenance. It considers the manifestations of fossilization and attrition as well as their indications as to the linguistic competences of bilingual users. It eventually attempts to estimate how widespread the two phenomena may be in the bilingual context and what their product is.