Jets are one of the classic manifestations of the active galactic nucleus (AGN) phenomenon, with the first example having been discovered in 1918:  what Heber Curtis described as a "curious straight ray" in the galaxy M87.  Today we know that jets are high-energy streams of matter and energy that emerge from the nuclear regions at relativistic speeds and have emissions that are seen from the radio through the gamma-rays.  The flows carry with them immense kinetic energy, comparable to the luminosity of the AGN itself, and are now understood to be a transformative force that feeds back into the evolution of the surrounding galaxy and cluster.  In this talk, I will concentrate on the physics of jets.  I will begin with a review of the field, focusing on structural elements, variability, and the production of high energy radiation.   I will then turn to the open questions, emphasizing the unique information that can be revealed by polarimetry.  I will then talk about HST polarimetry and Chandra X-ray observations of nearby jets, where we are learning new information about the jet structure and the role of magnetic fields in particle acceleration.  Of particular interest will be the M87 jet, where thanks to its proximity, we have been successful in for the first time isolating a violently variable region within the jet.  This gives us powerful insight into the physical processes that surround its variability and response to stimuli.