While native and non-native English-speakers may approach scientific English differently, the same features cause difficulty for both groups. The difficulties generally occur more with grammar and the complex relationships between terms than with vocabularly, and may be classified in seven categories: interlocking definitions, technical taxonomies, special expressions, lexical density, syntactic ambiguity, grammatical metaphor, and semantic discontinuity. These features are closely related and tend to cluster together as characteristics of scientific discourse. They are not arbitrary, but have evolved to meet the needs of scientific method, argument, and theory. As learners master these features, they are also mastering scientific concepts and principles. The best tool available for helping students understand scientific English is functional grammar, which enables analysis of any passage in relation to its context in the discourse. (MSE)