Understanding massive disk galaxy formation through resolved stellar populations In this thesis we utilize resolved stellar populations to improve our understanding of galaxy formation and evolution. In the first part we improve a method for metallicity determination of faint old stellar systems in the second and third part we analyze the individual history of six nearby disk galaxies outside the Local Group. A New Calibration of the Color Metallicity Relation of Red Giants for HST data: It is well known that the color distribution of stars on the the Red Giant Branch (RGB) can be used to determine metallicities of old stellar populations that have only shallow photometry. Based on the largest sample of globular clusters ever used for such studies we quantify the relation between metallicity and color in the widely used HST ACS filters F606W and F814W. We use a sample of globular clusters from the ACS Globular Cluster Survey and measure their RGB color at given absolute magnitudes to derive the color metallicity relation. We find a clear relation between metallicity and RGB color; we investigate the scatte…