This thesis investigates the abundance and properties of extremely red galaxies (EROs). Although the existence of such conspicuous galaxies has been known for some time, there are numerous open questions regarding their nature, formation, and evolution. One reason is the number of different colour criteria applied to select EROs. Hence, different galaxy populations are called "extremely red". In this thesis, galaxies with R-J≥5 are classified as extremely red. The available data set is based on optical (R-band) and near-infrared (J-band) observations. We have compiled a catalogue of 160 objects, which were classified as galaxies either due to their surface brightness distribution or their brightness. Their surface and co-moving volume density is compared to that of similar objects, such as galaxies with R-K≥7, and other, high-redshift galaxies. Since there are no spectra or spectral energy distributions of such galaxies available, we use stellar population synthesis models to predict which class of galaxies can satisfy our colour criterion. We test various star formation scenarios, both for elliptical galaxies, dusty starbursts and spiral galaxies. Only elliptical galaxies at redshift >1.4 and which have undergone the last phase of star formation at redshift z=3, have red enough spectral energy distributions to be classified as an ERO. Using multi-colour imaging on a small sample of fields, we have tested a photometric method to separate old elliptical galaxies and dusty starbursts with colours redder than R-K=5. Despite the small number of objects, we find that the relative abundance of both galaxy types is in good agreement with previously published results.