There has been a dramatic growth in interactive technology use by children under the age of 5 during the past decade. Despite this growth, children under the age of 5 typically participate only as users or testers in the design process in the overwhelming majority of projects targeting this population presented in key child-computer interaction venues. In this paper we introduce play-based design, an age-appropriate design method to give 3-4-year-old children a voice in the design process. More specifically, we contribute a thorough analysis of the use of existing methods to design technologies for children under the age of 5, a summary of the process that resulted in the development of play-based design, a detailed description of play-based design, a qualitative analysis of our experience implementing play-based design with two groups of children, and a discussion of play-based design's place among other methods, its advantages, and limitations.