Relativistic objects in astrophysics present a diverse set of phenomena and cover vast ranges of time scales and spatial scales, but their descriptions rely on a similar backbone: production of a relativistic outflow in a central compact object and dissipation of the outflow at large radii, accompanied by acceleration of particles up to very high energies and production of secondary non-thermal electromagnetic radiation. This book provides a timely and comprehensive review of the main physical processes involved in the formation and dissipation of relativistic outflows. It will be particularly useful to postgraduate students and researchers active in various areas of astrophysics and space science .Originally published in Space Science Reviews, Volume 207, Issue 1-4, July 2017