The region of the Hertzsprung-Russell diagram occupied by stars during their central H-burning phase is called the main sequence. It is a quasi-diagonal band, running from low to high values of stellar luminosities and effective temperatures. The position of a star in that band is mostly determined by its mass (through the mass-luminosity relation), and to a much smaller degree by its metallicity (where astronomers refer to all elements heavier than helium as “metals”; less metallic stars being hotter) and age (older stars being more luminous). Since H-burning (fusion of hydrogen to helium) is the longest period in a star’s life, about 90 % of all stars are on the main sequence. In star clusters, the upper end of the main sequence is truncated since the more massive stars have evolved toward the red giant branch (or already died); the position of this truncation provides a means to infer the cluster’s age.