The research considers the features and requirements for legal language and plain language with the purpose to analyse legalese in the context of plain language as a means to ensure clear expert-to-layman communication. The findings based on the analysis of the text of the Treaty of Lisbon show that many typical lexical features of legal language cause vagueness and impede the reader’s perception. Nowadays, the use of legalese is not restricted to the legal profession any more. Therefore, recently the principles of plain language have started being applied to official documentation with the emphasis on precision, clarity of expression, avoidance of unnecessary details, etc. The ever-increasing demand for clear expert-to-layman communication naturally increased the demand for intralingual translation by applying plain language principles to legal language.