This paper describes the development and use of MatML, the Materials Markup Language. MatML is an emerging XML standard intended primarily for the exchange of materials property information. It provides a medium of communication for users in materials science and related fields such as manufacturing and aerospace. It sets the stage for the development of semantic web standards to enhance knowledge discovery in materials science and related areas. MatML has been used in applications such as the development of materials digital libraries and analysis of contaminant emissions data. Data mining applications of MatML include statistical process control and failure analysis. Challenges in promoting MatML involve satisfying a broad range of constituencies in the international engineering and materials science community and also adhering to other related standards in web data exchange. These issues are being addressed through the development of a good ontology, the automation of format conversions and possible schema extensions. MatML aims to be a lingua franca for data exchange in materials science and its broader horizons.