Fuzzy L-systems are presented as a formal mechanism for generating fuzzy formal languages. This provides a mechanism for modelling development while also incorporating the vagueness inherent to most real world processes. In this paper, fuzzy L-systems are defined, a normal form is developed, closure properties of fuzzy L-systems—both language-theoretic and related to fuzziness—are explored, and some decidability issues are also considered. © 1990, Taylor & Francis Group, LLC. All rights reserved.