Julia is a programming language for the scientific community that combines features of productivity languages, such as Python or MATLAB, with characteristics of performance-oriented languages, such as C++ or Fortran. Julia has many productivity features: dynamic typing, automatic memory management, rich type annotations, and multiple dispatch. At the same time, it lets programmers control memory layout and uses a specializing just-in-time compiler that eliminates some of the overhead of those features. This paper details these choices, and reflects on their implications for performance and usability.