Contents: (1) Introduction: Basic data. (2) Optical and infrared observations: Doppler-shifted spectral lines. Deviations from the kinematic model. The stationary spectrum and "normal" star. Interstellar lines and extinction. Optical and infrared photometry. (3) Radio observations: The central source. The extended components. (4) X- and gamma-ray observations. (5) W50. (6) Some major uncertainties: Jet energetics, collimation, and acceleration. The disk and precession mechanism. The nature of the compact star. The extragalactic analogy. (7) Analogous galactic objects.