For the study of galaxies, a foundational knowledge of basic cosmology is essential, as the observed data must be interpreted through theoretical models. Although the primary epoch of cosmic history occurs after decoupling, it is crucial to understand the entire history of the Universe. We begin by introducing the fundamental cosmological tools necessary to describe cosmic expansion. Next, we derive essential quantities in observational cosmology, including luminosity and angular diameter distances, volume elements, and the size of the particle horizon. Using these formulas, we explore various observables such as flux density, surface brightness, and the correction of flux density and wavelength with redshift (K-correction), among others. In the context of element production within galaxies, a detailed examination of Big-Bang Nucleosynthesis is provided. Furthermore, some fundamental aspects of the cosmic microwave background (CMB) are introduced.