Publisher Summary This chapter focuses on the lifetime measurements of free atoms, molecules, and ions because it is well-known that atoms, ions, and molecules in excited electronic levels tend to decay spontaneously to low energy levels of their respective systems by the emission of electric dipole radiation. The simplest experiment that can be imagined to measure lifetimes would consist of a sample chamber in which the atoms to be studied are excited to the desired upper level by electron bombardment. In most multichannel systems, the lifetime resolution is limited by the combined effects of the finite time taken for the gun pulse cutoff and the finite transit time of electrons across the source. The numerical methods discussed in this chapter can be used to resolve the observed decay curves into a sum of exponential components. However, considerable care is required in analyzing and interpreting the results obtained from cascade decay curves, and the possibility exists of making quite serious errors. The lifetime has also been obtained indirectly by the combined studies of the intensities of absorption and emission spectra.