Ladder Materials --- Lagrange Multipliers --- Lagrange's Equations --- Lambda Bootis Stars --- Landau-Ginzburg Models --- Landmarks --- Large Hadron Collider --- Large N Expansion --- Large Scale Structure (Astrophysical) -- Larmor Precession --- Larmor Theorem --- Lasers --- Lattice Boltzmann Equation --- Lattice QCD --- Lax Operators --- Lee-Friedrichs Model --- Leeson Effect --- Left-Handed Materials --- Legendre Transform --- Lense-Thirring Effect --- Leptogenesis --- Lepton Flavor Oscillations --- Lepton Colliders --- Lepton Number --- Leptons --- Leptoquarks --- Liaison Theory --- Lie Algebras --- Ligands --- Light Cone Physics --- Light-Emitting Diodes --- Light Mesonic Resonances --- Lightning --- Liquid Crystals --- Liquid Mirrors --- Little Group --- Little Higgs --- Local Group --- Local Sheet and Void --- Localization --- Logic --- London Model --- Loop Quantum Gravity --- Lorentz Violation --- Low Surface Brightness Galaxies --- Low-Temperature Physics --- Low X Physics --- Lubrication --- Luminescence --- Luminosity --- Luttinger Fluids --- Lyman Alpha Absorbers --- Lyman Alpha Emitters