The spectral distribution of light scattered by density fluctuations in a dense, monatomic, one-component fluid is calculated from the time dependence of the density fluctuations predicted by the linearized hydrodynamic equations of irreversible thermodynamics. The results of Landau and Placzek are verified and a procedure for deriving correction terms is discussed with the dispersion in the velocity of thermal sound waves obtained as an illustration. Particular attention is paid to the critical region. The properties of carbon dioxide are used to estimate the spectral distribution of critical opalescence. A comparison is made between light-scattering and sound-propagation experiments. Space dispersion near the critical point in the pressure and the thermal conductivity is examined briefly. Finally, some of the experimental problems involved in measuring the spectral distribution of the scattered light are discussed.