The article reviews the 25-year journey of the amyloid hypothesis in Alzheimer's disease (AD). Despite ongoing debates, extensive evidence from preclinical and clinical studies supports the concept that an imbalance in the production and clearance of amyloid β-protein (Aβ) is a key early factor in AD. Key findings include: 1. **Genetics and APP Homeostasis**: Mutations in APP and presenilins alter Aβ processing, leading to increased Aβ42/Aβ43 ratios and subsequent neurotoxicity. Down syndrome, characterized by trisomy 21, demonstrates the lifelong overexpression of APP and the resulting Aβ deposition. 2. **Apolipoprotein E (ApoE)**: ApoE4 is a major risk factor for AD, influencing Aβ clearance and contributing to AD risk through differential regulation of Aβ levels. 3. **Cell Biology and Risk Genes**: Recent studies have identified risk loci for late-onset AD, including those involved in cholesterol metabolism, inflammation, and endosomal vesicle recycling. These processes are linked to Aβ homeostasis and may provide new therapeutic targets. 4. **Biomarkers and Clinical Trials**: Advances in biomarker detection and imaging have helped identify the sequence of Aβ and tau accumulation in AD. Clinical trials of Aβ-targeted therapies, such as solanezumab, crenezumab, and aducanumab, have shown promising results in slowing cognitive decline in mild AD subjects. 5. **Heterogeneity of Aβ Species**: The complexity of Aβ species in AD brains, including modified and extended forms, highlights the need for routine quantification of all Aβ peptides in plasma or CSF. 6. **Prion-like Spread**: The hypothesis of prion-like spread of misfolded proteins in AD is supported by some experimental and clinical observations, though the exact mechanisms remain unclear. The article concludes by emphasizing the compelling nature of the evidence supporting the amyloid hypothesis and the ongoing efforts to develop effective treatments targeting Aβ dyshomeostasis.The article reviews the 25-year journey of the amyloid hypothesis in Alzheimer's disease (AD). Despite ongoing debates, extensive evidence from preclinical and clinical studies supports the concept that an imbalance in the production and clearance of amyloid β-protein (Aβ) is a key early factor in AD. Key findings include: 1. **Genetics and APP Homeostasis**: Mutations in APP and presenilins alter Aβ processing, leading to increased Aβ42/Aβ43 ratios and subsequent neurotoxicity. Down syndrome, characterized by trisomy 21, demonstrates the lifelong overexpression of APP and the resulting Aβ deposition. 2. **Apolipoprotein E (ApoE)**: ApoE4 is a major risk factor for AD, influencing Aβ clearance and contributing to AD risk through differential regulation of Aβ levels. 3. **Cell Biology and Risk Genes**: Recent studies have identified risk loci for late-onset AD, including those involved in cholesterol metabolism, inflammation, and endosomal vesicle recycling. These processes are linked to Aβ homeostasis and may provide new therapeutic targets. 4. **Biomarkers and Clinical Trials**: Advances in biomarker detection and imaging have helped identify the sequence of Aβ and tau accumulation in AD. Clinical trials of Aβ-targeted therapies, such as solanezumab, crenezumab, and aducanumab, have shown promising results in slowing cognitive decline in mild AD subjects. 5. **Heterogeneity of Aβ Species**: The complexity of Aβ species in AD brains, including modified and extended forms, highlights the need for routine quantification of all Aβ peptides in plasma or CSF. 6. **Prion-like Spread**: The hypothesis of prion-like spread of misfolded proteins in AD is supported by some experimental and clinical observations, though the exact mechanisms remain unclear. The article concludes by emphasizing the compelling nature of the evidence supporting the amyloid hypothesis and the ongoing efforts to develop effective treatments targeting Aβ dyshomeostasis.