The chapter discusses how twilight and day airglow are produced through resonance scattering and fluorescence. Observations of sodium day airglow from balloons provide important information on the sodium layer. The abundance of Na atoms in the airglow helps understand the total sodium abundance, with sodium being incorporated into compounds at low altitudes and ionized at higher levels. The intensity of dayglow is similar to twilight, and the Na abundance remains relatively constant throughout the day and night based on observations.