The phenomenon of peptide and protein aggregation is of great biochemical importance. Not only is it a key feature of numerous neurodegenerative diseases such as Alzheimer's, Parkinson's or Huntington's disease, but it also plays a significant role in the manufacturing and processing of therapeutic peptides. In the process of producing pharmaceutical peptides, molecules are exposed to a wide variety of conditions, some of which enhance aggregation, such as high peptide concentrations. In addition, the long-term physical stability of therapeutic peptides is essential for storage and use of these drugs, and also influences their half-lives in vivo. Therefore, aggregation of peptide-based therapeutics remains a great challenge for the pharmaceutical industry. Aggregation is typically defined as a process involving the non-covalent association of polypeptide chains, however, in some cases aggregates can also be linked covalently. The process of aggregation proceeds via a multi-step mechanism: First, the monomeric peptide self-associates to oligomeric structures, and these can form nucleating species which then further elongate to form amyloid fibrils. When a peptide aggregates, it leads not only to the loss of its biological activity but it can also give rise to other critical problems, e.g. toxicity and immunogenicity, associated with the formation of intermediate oligomeric species. This work is focused on the C-terminal amidated analogue of the commonly used peptide therapeutic, glucagon-like peptide 1 (GLP-1). This peptide is responsible for many regulatory mechanisms affecting the level of glucose in the bloodstream. Unfortunately, it was shown that GLP-1 has a great propensity to aggregate over a wide pH range that inhibits its function. The studies presented here establish that C-terminal amidation of this peptide hormone significantly slows its fibril formation at neutral and basic pH. It was found that during the significantly prolonged lag times, small stable soluble peptide oligomers start to form. These oligomers, which were characterised using a number of different biophysical techniques, may be off-pathway species of amyloid fibril formation or products of partial peptide degradation. The aggregation kinetics were shown to differ below and above pH 8. However, it was demonstrated that oligomers formed during the process have similar structural characteristics. While amyloid fibrils have a characteristic cross β-sheet structure, the structure of small oligomers formed mainly during the lag and elongation and growth phases is highly disordered. Surprisingly, these small oligomers show a great stability. The exact role of these small, soluble disordered aggregates in fibrillation processes requires further investigation. These results contribute to the understanding of a complex pathway of aggregation and misfolding processes by providing size and structural characterization of oligomeric species formed during the lag, elongation and growth phases.