Proteins are a basic building block built by connecting 20 different amino acids in the polypeptide chain. Each of the 20 amino acids differs in properties due to their structure or due to different amino acid residue. Amino acids are linked together with peptide bonds and form four levels in protein structure: primary, secondary, tertiary and quaternary. Protein properties and the protein stability itself depends on sequence of the amino acids residues that make up a particular protein. Since 1986, when the N-rule rule has been set up, studies into eukaryotes and prokaryotes are conducted to get more detailed data on protein stability. The N-rule indicates the stability of a given protein and how long its half-life is. The two branches of the N-rules in the eukaryotes are the Arg/N rule and the Ac/N rule. Each branch has its own way of recognizing proteins for ubiquitination. The N-end rule has determined the functions of many proteins and enzymes. Ubiquitinated proteins are degraded proteolytically in the proteosome. Prokaryotic cells have a similar way of recognizing proteins, but a different way of marking without ubiquitin and the proteolytic protein complex is working by a different mechanism. This paper will briefly explain the basics of proteins, their stability dependent on aminoacid residues, the N-line rule, its branch, function, and the mechanism of degradation of proteins in eukaryotes and prokaryotes.