The formation of amyloid aggregates from misfolded proteins is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Misfolded protein oligomers, which are intermediate species produced during amyloid formation, are increasingly recognized as the primary cytotoxic agents in these conditions. This review examines the processes by which oligomers form, their structures, physicochemical properties, population dynamics, and mechanisms of cytotoxicity. It also discusses drug discovery strategies aimed at disrupting oligomer formation and their ability to disrupt cell physiology and trigger degenerative processes. Oligomers are smaller, intermediate, and metastable soluble aggregates that can be more toxic than mature fibrillar aggregates in many protein misfolding diseases. They can be cytotoxic by sequestering functional proteins or by directly damaging cells and tissues. Oligomer formation involves a series of intermediate states, including protofibrils, pentamers, and diffusible ligands. The kinetics of amyloid fibril formation can be analyzed using macroscopic measurements and kinetic models, which help understand the mechanisms of oligomer generation and their population dynamics. Recent advances in experimental methods have facilitated the detection and quantification of oligomers, enabling the development of kinetic models that explicitly include oligomer dynamics. These models can be used to investigate the effects of potential therapeutics or additive agents on targeting specific microscopic steps in the aggregation reaction. The role of oligomers in protein misfolding diseases is supported by clinical trials and in vitro studies, particularly for Alzheimer's disease, where lecanemab, an antibody targeting high molecular weight Aβ oligomers, has shown efficacy in slowing cognitive decline. However, direct evidence of a causal role of oligomers in disease remains challenging due to their small size, low stability, and transient nature. Oligomers have been shown to interact with biological membranes, specific membrane receptors, and cytosolic proteins, leading to a range of dysfunctional cellular interactions. The toxicity of oligomers is influenced by their size, hydrophobicity, and ability to disrupt membranes, with smaller, more hydrophobic oligomers being more cytotoxic. Overall, the review highlights the importance of understanding the formation and toxicity of misfolded protein oligomers to develop effective therapeutic strategies for protein misfolding diseases.The formation of amyloid aggregates from misfolded proteins is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Misfolded protein oligomers, which are intermediate species produced during amyloid formation, are increasingly recognized as the primary cytotoxic agents in these conditions. This review examines the processes by which oligomers form, their structures, physicochemical properties, population dynamics, and mechanisms of cytotoxicity. It also discusses drug discovery strategies aimed at disrupting oligomer formation and their ability to disrupt cell physiology and trigger degenerative processes. Oligomers are smaller, intermediate, and metastable soluble aggregates that can be more toxic than mature fibrillar aggregates in many protein misfolding diseases. They can be cytotoxic by sequestering functional proteins or by directly damaging cells and tissues. Oligomer formation involves a series of intermediate states, including protofibrils, pentamers, and diffusible ligands. The kinetics of amyloid fibril formation can be analyzed using macroscopic measurements and kinetic models, which help understand the mechanisms of oligomer generation and their population dynamics. Recent advances in experimental methods have facilitated the detection and quantification of oligomers, enabling the development of kinetic models that explicitly include oligomer dynamics. These models can be used to investigate the effects of potential therapeutics or additive agents on targeting specific microscopic steps in the aggregation reaction. The role of oligomers in protein misfolding diseases is supported by clinical trials and in vitro studies, particularly for Alzheimer's disease, where lecanemab, an antibody targeting high molecular weight Aβ oligomers, has shown efficacy in slowing cognitive decline. However, direct evidence of a causal role of oligomers in disease remains challenging due to their small size, low stability, and transient nature. Oligomers have been shown to interact with biological membranes, specific membrane receptors, and cytosolic proteins, leading to a range of dysfunctional cellular interactions. The toxicity of oligomers is influenced by their size, hydrophobicity, and ability to disrupt membranes, with smaller, more hydrophobic oligomers being more cytotoxic. Overall, the review highlights the importance of understanding the formation and toxicity of misfolded protein oligomers to develop effective therapeutic strategies for protein misfolding diseases.