As co-founders and co-directors of Jupyter, the two of us have been asked to introduce Jupyter to readers of this CISE issue, which represent a small fraction of the content from JupyterCon 2020. At the same time, 2021 marks the 10th anniversary of the Jupyter Notebook and the 20th anniversary of IPython. As such, we believe it is worth pausing and asking two questions. What are the main ideas of Jupyter and why have Jupyter Notebooks turned out to be so useful to such a wide range of users and domains?