Published Article / This article reviews research on learning with external representations and provides a theoretical background on how to optimize learning from external representations. General factors, such as the type of material to be learned, learner characteristics and the testing method, are some of the variables that can determine if graphic medium can increase a subject's comprehension and if such comprehension can be accurately measured. These factors are discussed and represented by a model to suggest how external representations can be effectively used in a learning environment. Two key conclusions are drawn from the observation made in these studies. Firstly, the proper design of a particular external representation and supporting text can promote relevant activities that ultimately contribute to fuller understanding of the content. Secondly, external representations must be developed to address the size complexity and variety of the content that must be analysed in order to extract knowledge for scientific discovery.