1 The Hanle Effect and Level-Crossing Spectroscopy-An Introduction.- 1. Historical Survey.- 2. Classical Interpretation of the Hanle Effect.- 3. Quantum Mechanical Interpretation of the Hanle Effect.- 4. The Density Matrix Formalism for the Hanle Effect (Broad-Band Excitation).- 5. Laser Excitation and Pressure-Induced Coherences.- 6. Nonzero-Field Level Crossing.- 7. Conclusions.- References.- Appendix. Magnetic Effects on the Polarization of Resonance Fluorescence (original work by Wilhelm Hanle, translated by G. Moruzzi).- 2 The Hanle Effect and Atomic Physics.- 1. Introduction.- 1.1. General Expression for the Hanle Signal in Terms of the Density Matrix.- 2. Spectroscopic Applications.- 2.1. Determination of Atomic Constants.- 2.2. Measurements of Laser-Level Populations.- 2.3. Increasing Resolution, Subnatural Linewidth Effects.- 2.4. Forward Scattering, Line Crossing.- 2.5. Technical Applications.- 3. Collisions.- 3.1. Hanle Effect with Collisional Excitation.- 3.2. Hanle Effect and Optogalvanic Detection.- 3.3. Collision-Induced Hanle Resonances.- 3.4. Fluctuation-Induced Hanle Resonances.- 4. Hanle Effect in Strong Laser Fields.- 4.1. General Characteristic.- 4.2. Specific Situations.- 4.3. Hanle Effect and Nonlinear Optics.- 5. Hanle Effect in Quantum Optics.- 5.1. Dressed-Atom Model.- 5.2. Hanle Effect with Fluctuating Fields.- 5.3. Squeezing in the Hanle Effect.- References.- 3 The Hanle Effect and Level-Crossing Spectroscopy on Molecules.- 1. Introduction.- 2. Molecular Level-Crossing Signal.- 3. Comparison with Quantum Beat Experiments.- 4. Excitation of Molecules.- 5. Lifetime Investigations.- 6. Landé g-Factors.- 7. Electric-Field Level Crossing.- 8. Stark-Zeeman Recrossing and High-Field Level Crossing.- 9. Hanle Effect on NO2.- 9.1. The Influence of Detection Geometry.- 9.2. Details of the Hanle-Effect Signal.- 9.3. Collisions.- 9.4. Discussion of Hanle-Effect Experiments on NO2.- 10. Conclusion.- References.- 4 The Nonlinear Hanle Effect and Its Applications to Laser Physics.- 1. The Nonlinear Hanle Effect and Its Experimental Observation.- 2. Saturation Intensity and Saturated Linewidth.- 3. The Three-Level Case: Homogeneously Broadened Lines.- 4. The Three-Level Case: Doppler-Broadened Lines and the Rate Equations.- 5. The General Case.- 6. The Rate-Equation Approach to the Nonlinear Hanle Effect in Inhomogeneously Broadened Transitions.- 7. The Nonlinear Hanle Effect with a Gaussian Laser Beam.- 8. The Nonlinear Hanle Effect in Absorption.- 9. The Nonlinear Hanle Effect in Laser-Active Media.- 9.1. The He-Ne Laser.- 9.2. The Xe Laser.- 9.3. The He-CdII and He-ZnII Lasers.- 9.4. The Noble-Gas Ion Lasers.- 9.5. Optically Pumped Far-Infrared Lasers.- 9.6. Other Lasers.- 9.7. Conclusions.- References.- 5 Applications of the Hanle Effect in Solar Physics.- 1. Introduction.- 2. Brief Review of the Properties of Solar Magnetic Fields.- 3. Overview of the Diagnostic Possibilities and Limitations of the Hanle Effect.- 4. Basic Theoretical Concepts for Applications in Astrophysics.- 5. Diagnostics of Magnetic Fields in Solar Prominences.- 6. Survey of Scattering Polarization on the Solar Disk.- 7. Diagnostics of Turbulent Magnetic Fields.- 8. Diagnostics of Magnetic Fields in the Chromosphere-Corona Transition Region and Above.- 9. Concluding Remarks.- References.- 6 Applications of the Hanle Effect in Solid State Physics.- 1. Introduction.- 2. The Hanle Effect on Free Electrons.- 2.1. Optical Orientation of Electron Spins.- 2.2. Occurrence of Electron-Nucleus Interaction in Polarized Luminescence.- 2.3. Optical Alignment of Electron Momenta in a Magnetic Field.- 3. The Hanle Effect on Excitons.- 3.1. The ?8 × ?6 and ?7 × ?6 Excitons in Cubic Crystals.- 3.2. The ?9 × ?7 and ?7 × ?7 Excitons in Hexagonal II-VI Crystals with Wurtzite Structure.- 3.3. The ?7 × ?8 Excitons in III-VI Crystals with Symmetry Class D3h.- 3.4. The Influence of Reemission on the Hanle Effect.- 3.5. Hot Excitons and Polaritons.- 4. The H