Weld is an open-source Rust project for improving the performance of data-intensive applications. It is an interface and runtime that can be integrated into existing frameworks including Spark, TensorFlow, Pandas, and NumPy without changing their user-facing APIs. Data analytics applications today often require developers to combine various functions from different libraries and frameworks to accomplish a particular task. For instance, a typical Python ecosystem application selects some data using Spark SQL, transforms it using NumPy and Pandas, and trains a model with TensorFlow. This improves developers' productivity as they are taking advantage of functions from high-quality libraries.