In-space propulsion systems is a field within aerospace engineering and physics dedicated to the research, design, development, and application of technologies that generate thrust for spacecraft *in vacuo*. This area investigates the principles and methodologies for achieving controlled changes in velocity (delta-v) and orientation (attitude control) necessary for orbital maneuvers, station keeping, interplanetary transit, and deep space exploration, emphasizing characteristics such as high specific impulse, efficiency, mass constraints, and reliability under space conditions.