Each big journey starts with a first step: Importance of oligomerization.

Protein oligomers, widely found in nature, have significant physiological and pathological functions. They are classified into three groups based on their function and toxicity. Significant advancements are being achieved in the development of functional oligomers, with a focus on various applications and their engineering. The antimicrobial peptides oligomers play roles in death of bacterial and cancer cells. The predominant pathogenic species in neurodegenerative disorders, as shown by recent results, are amyloid oligomers, which are the main subject of this chapter. They are generated throughout the aggregation process, serving as both intermediates in the subsequent aggregation pathways and ultimate products. Some of them may possess potent cytotoxic properties and through diverse mechanisms cause cellular impairment, and ultimately, the death of cells and disease progression. Information regarding their structure, formation mechanism, and toxicity is limited due to their inherent instability and structural variability. This chapter aims to provide a concise overview of the current knowledge regarding amyloid oligomers.