Longitudinal Waves: Particles oscillate back and forth, parallel to the wave's direction. Consists of compressions (high-pressure, dense regions) and rarefactions (low-pressure, spread-out regions). Require a solid, liquid, or gas medium to travel; cannot move through a vacuum. Examples are sound waves, seismic P-waves, ultrasonic waves. Transverse Waves: Particles oscillate up and down (or side-to-side), perpendicular (at right angles) to the direction of energy transfer. Comprised of crests (h