The Hertzsprung-Russell diagrams of low metallicity globular clusters provide excellent demonstrations of the asymptotic giant branch, or AGB. The HR-diagram of M3, shown in Figure 2.1, is a good example. When HR-diagrams such as Figure 2.1 were first obtained in the 1950s, the asymptotic merging of the AGB and the red giant branch (RGB) was regarded as a “bifurcation” viewed looking down from the tip of the giant branch (e.g., Sandage [123]; Arp [7]). At this stage the direction of evolution was unknown. Probably the first authors to use the term “asymptotic” were Sandage and Walker [124]. This was just after Schwarzschild and Härm [129] had evolved a low mass star past the Horizontal Branch and up the AGB. Although the term asymptotic giant branch originated as a description of the sequence of stars in the HR diagrams of low-mass globular cluster stars, the term is now generally used to describe all stars with masses ≲ 8 M⊙ that are on the second ascent into the red giant region of the HR-diagram.