The Casimir effect arises because quantum vacuum fluctuations of the electromagnetic field are suppressed between two parallel conducting plates — only modes with wavelengths that fit an integer number of half-wavelengths between the plates are permitted. The resulting radiation pressure imbalance pushes the plates together with force F = −π²ℏc·A / (240 d⁴). At 100 nm separation this force reaches ~1 mPa, measurable with atomic force microscopy. Zeta regularization (ζ(−3) = 1/120) is used to sum the divergent zero-point energies and extract the finite physical result.