In this post we’d like to make a major announcement about where PyMC is headed, how we got here, and what our reasons for this direction are. TL;DR: PyMC3 on Theano with the new JAX backend is the future, PyMC4 based on TensorFlow Probability will not be developed further. In 2017, the original authors of Theano announced that they would stop development of their excellent library. This left PyMC3