I. Basic Observational Data.- 1. Forms of planetary nebulae.- 2. The structure of nebulae.- 3. Luminosity of nebulae and their nuclei.- 4. The spectra of nebulae.- 5. The spectra of the nuclei.- 6. Stratification of radiation.- 7. Expansion of planetary nebulae.- 8. Apparent distribution of planetary nebulae.- 9. Planetary nebulae in other galaxies.- 10. Spectral and morphological classification of nebulae.- 11. Designations and catalogues.- II. The Origin of Emission Lines.- 1. The nature of nebular emission.- 2. Rosseland's theorem.- 3. Fluorescence-Zanstra's theory.- 4. Excitation of the forbidden lines.- 5. Accumulation of atoms in metastable levels.- 6. The role of electron collisions.- 7. The stratification of radiation.- 8. The intensity of the Balmer lines.- 9. Degree of excitation.- 10. Ionization in the nebula.- 11. Ionization and excitation by electronic collisions.- 12. Chemical composition of planetary nebulae.- 13. Continuous absorption coefficients of several elements.- III. The Problem of Distances and Dimensions of Planetary Nebulae.- 1. The state of the problem.- 2. Methods of determining distances of planetary nebulae.- 3. The dispersion of luminosity and mass in planetary nebulae.- 4. Astrophysical methods of determining the distances of nebulae.- IV. Temperatures of Nuclei, Electron Temperatures and Electron Concentrations.- 1. Methods of determining temperatures of nuclei.- 2. Electron temperatures.- 3. Cooling of the inner regions of nebulae.- 4. The temperature gradients in nebulae.- 5. The possibility of determining the electron temperature of the central regions of nebulae from observations.- 6. The electron concentration.- 7. Planetary nebulae with very high densities.- 8. The masses of planetary nebulae.- V. The Continuous Spectrum ofPlanetary Nebulae.- 1. Observational data.- 2. Theory of two-quantum emission.- 3. The effects of collisions.- 4. Processes degrading L?-photons.- 5. The intensity of continuous radiation.- 6. Continuous emission of unknown origin.- 7. The possibility of synchrotron radiation.- 8. The possibility of radio emission.- VI. Radiative Equilibrum of Planetary Nebulae.- 1. The Lc-radiation field.- 2. The L?-radiation field.- 3. The radiation field in an expanding nebula.- 4. Diffusion of L?-radiation.- 5. Relaxation time.- 6. Radiation pressure in nebulae.- 7. The expansion of nebulae.- 8. The effect of thermal expansion. "Vacuum" in the centers of nebulae.- 9. Deceleration of nebulae.- VII. Double-Envelope Nebulae.- 1. Observations of double-envelope nebulae.- 2. Basic properties.- 3. The origin of the second envelope.- 4. The evolution of planetary nebulae.- 5. The theory of envelope separation.- 6. On the optical depth.- 7. Ring nebulae.- VIII. The Stability of the Forms of Gaseous Envelopes.- 1. The hydrodynamics of planetary nebulae.- 2. Statement of the problem.- 3. The stability of the forms of gaseous envelopes.- 4. The stability of planetary nebulae.- 5. The stability of nova envelopes.- 6. Stability of the envelopes of Wolf-Rayet stars.- 7. Application of the theory to duffuse nebulae.- 8. The influence of macroturbulence on the stability of the envelopes.- 9. On the motion of planetary nebulae in the interstellar medium.- IX. Magnetic Fields in Planetary Nebulae.- 1. The role of Galactic magnetic fields.- 2. Mechanical equilibrium in planetary nebulae with a magnetic field.- 3. Energy equilibrium of planetary nebulae.- 4. A point dipole.- 5. An extended dipole.- 6. Brightness distribution of a nebula with a magnetic field.- 7. Elongated nebulae.- 8."Rectangular" nebulae.- 9. The magnetic field of NGC7293.- 10. Spiral planetary nebulae.- 11. Dissipation of magnetic energy.- 12. The interstellar magnetic field of the intermediate Galactic system.- 13. Some conclusions.- X. The Origin of Planetary Nebulae.- 1. The ejection hypothesis.- 2. The "residue" theory.- 3. The early age of the nuclei.- 4. The evolution of nuclei.- 5. The temperature of IC4997.- 6. The contribution of continuous ejection (continued).- 7. Rate of ejection of gaseous matter by Wolf-Rayet stars.- 8. Former nuclei of planetary nebulae.- 9. Hot stars at high Galactic latitudes.- 10. Humason-Zwicky objects.- 11. The origin of diffuse nebulae.- 12. The cosmogonic significance of planetary nebulae.- References.