Reward systems and reinforcement design are the systematic application of behavioral psychology principles to shape behavior through incentives and consequences. They operate across organizational, educational, digital, and consumer contexts—from employee recognition programs to habit-tracking apps to customer loyalty schemes. The core challenge isn't whether to reward behavior, but how: intrinsic motivation can be undermined by poorly designed extrinsic rewards, variable schedules create stronger habits than predictable ones, and the timing and salience of a reward often matter more than its magnitude. Understanding the neuroscience of dopamine, the perils of hedonic adaptation, and the strategic use of gamification elements allows designers to build systems that sustain motivation rather than exhaust it.