Crystal Toolkit is a Python library and web application framework from the Materials Project for building interactive, browser-based visualizations and analysis tools for crystal structures and materials data. Built on Plotly Dash and pymatgen, it provides a set of reusable components (e.g., structure viewer, phase diagram, band structure, X-ray diffraction, Pourbaix diagrams, symmetry analysis) that can be combined into custom dashboards or used as standalone apps. The core architecture centers around an `MPComponent` base class that manages Dash stores, callbacks, and layout generation, and a `Scene` graph for 3D rendering (consumed by a Three.js-based component). The toolkit also includes utilities for export (Asymptote, POV-Ray) and JupyterLab integration via a MIME type extension. While primarily a library for application developers, it is tightly integrated with the Materials Project API and pymatgen for materials science data, making it a bridge between research data and interactive web UIs.