In Node.js, unhandled promise rejections occur when a promise is rejected but no `.catch()` block or `try/catch` is used to handle the error. This can cause your application to crash, especially in Node.js versions where unhandled rejections are treated as fatal exceptions. To prevent this, you should always ensure that every promise has an error handler. If you are using `async/await`, wrap your code in `try/catch` blocks to handle rejections. For promise chains, always append a `.catch()` to the end of the chain. You can also use global error handling mechanisms like `process.on('unhandledRejection', ...)` to catch unhandled promise rejections and log or handle them appropriately, though it’s recommended to catch errors at the source. By consistently handling rejected promises, you can prevent crashes and ensure your app behaves predictably in error scenarios.