Geodesic deviation is described by the Jacobi equation: D²ξ/dτ² = R(u,ξ)u, where ξ is the separation vector between nearby geodesics and R is the Riemann curvature tensor. In positively curved space (like a sphere), initially parallel geodesics converge — this is the tidal focusing that leads to gravitational lensing. In negatively curved (hyperbolic) space, geodesics diverge exponentially. The geodesic deviation equation encodes the entire content of the Einstein field equations when applied to families of free-falling observers, making it central to understanding tidal forces and spacetime singularities.