The quantitative description of atomic radiation reviewed in the previous chapter requires knowledge of energy levels and of level populations, which will be the principal subjects of chapters 6 and 7, and of oscillator strengths or of the closely related line strengths. Their calculation and measurement are the main topics of the present chapter. As to energy levels and wavelengths, a large body of high quality empirical data is available (Moore 1949-1958, Martin, Zalubas and Hagen 1978, Cowan 1981, Kelly 1987a and b, Bashkin and Stoner 1975, 1978, 1981), and atomic structure calculations (Cowan 1981, Sobel'man 1992) are usually of an accuracy that is sufficient for most plasma spectroscopy applications. Our knowledge of oscillator strengths, and therefore also of transition probabilities for spontaneous transitions and line strengths, has also greatly improved since the predecessor of this monograph (Griem 1964) was written. These advances are the result of improved experiments and computations and of critical evaluations of data (Wiese, Smith and Glennon 1966, Wiese, Smith and Miles 1969, Fuhr and Wiese 1995, Martin, Fuhr and Wiese 1988, Fuhr, Martin and Wiese 1988). Especially complete and accurate data are now available for atoms and ions of carbon, nitrogen and oxygen (Wiese, Fuhr and Deters 1995).