Spacecraft have explored the interplanetary medium between the orbits of Mercury and Pluto, revealing the exotic and beautiful nature of the environment of our planet and sun. This book summarizes the principle results of these historic expeditions. Magnetohydrodynamics provides the framework for interpreting the observations, and the observations have greatly enriched the subject of magnetohydrodynamics. Numerous figures in the book illustrate the essential observations and fundamental concepts. Simple equations summarize basic physical and phenomenological relations observed in the interplanetary medium. Fundamental objects observed in the interplanetary medium include various types of MHD shocks, tangenital and rotational discontinuities, and force-free field configurations. Fundamental processes observed in the interplanetary medium include the interaction among shocks (which can be classified using catastrophic theory), the formation of merged interaction region regions associated with various types of flows, the destruction of flows, the growth of the Kelvin-Helmholtz instability and formation of a heliospheric vortex street, the development of multifractal fluctuations on various scales, and the evolution of multifractal intermittent turbulence. These physical objects and processes are of fundamental interest in themselves. The knowledge about them derived from interplanetary investigations has applications to magnetospheric physics, solar physics, cosmic ray physics and astrophysics. The book contains an extensive set of reference that gives the readers access to the detailed results in the literature.