The cellular phase of Alzheimer's disease (AD) involves complex interactions among astrocytes, microglia, and the vasculature, leading to progressive neurodegeneration. The amyloid hypothesis, which posits that amyloid-beta (Aβ) plaques directly cause AD, is challenged by evidence showing that Aβ is part of a broader, more complex cellular process. This process includes feedback and feedforward mechanisms that initially respond to Aβ and Tau pathology but eventually lead to irreversible neurodegeneration. The amyloid cascade hypothesis, while influential, is criticized for oversimplifying the disease's progression and failing to account for the long incubation period and the role of other factors like Tau pathology. The biochemical phase of AD involves the aggregation of Aβ and Tau, leading to proteostatic stress and homeostatic cellular responses. These responses include autophagy, which is crucial for clearing Aβ and Tau. However, defects in autophagy can contribute to neurodegeneration. The cellular phase of AD is characterized by the interplay between different cell types, including astrocytes, microglia, and oligodendrocytes, which contribute to the disease's progression over decades. The transition from reversible to irreversible cellular responses is a critical phase in AD. Defective clearance mechanisms, such as impaired Aβ and Tau removal, are part of the initial cellular phase. The neurovascular unit, including the blood-brain barrier, plays a significant role in AD, with vascular damage contributing to the disease. The vascular hypothesis suggests that initial vascular damage leads to AD, with implications for Aβ and Tau metabolism and clearance. Neurons and neuronal circuits are central to the cellular phase of AD, with Aβ stress affecting synaptic plasticity and leading to excitatory and inhibitory network imbalances. Astrocytes are crucial in the cellular phase, involved in Aβ catabolism, synaptic support, and modulation of neuronal activity. Reactive astrocytes can contribute to neurodegeneration through impaired clearance and altered signaling. Microglia, the immune cells of the CNS, play a complex role in AD, with both beneficial and harmful effects. TREM2 and CD33 are key genes associated with microglial function and AD risk. Oligodendrocytes, which support myelination and axonal function, are also involved in AD, with myelin breakdown contributing to neurodegeneration. Systems biology approaches highlight the complexity of AD, with multiple pathways and interactions. Single-cell studies provide insights into the heterogeneity of cellular responses in AD, emphasizing the need for cellular resolution in understanding the disease. The cellular phase of AD is a dynamic process involving multiple cell types and pathways, with the transition from reversible to irreversible responses being a critical factor in disease progression.The cellular phase of Alzheimer's disease (AD) involves complex interactions among astrocytes, microglia, and the vasculature, leading to progressive neurodegeneration. The amyloid hypothesis, which posits that amyloid-beta (Aβ) plaques directly cause AD, is challenged by evidence showing that Aβ is part of a broader, more complex cellular process. This process includes feedback and feedforward mechanisms that initially respond to Aβ and Tau pathology but eventually lead to irreversible neurodegeneration. The amyloid cascade hypothesis, while influential, is criticized for oversimplifying the disease's progression and failing to account for the long incubation period and the role of other factors like Tau pathology. The biochemical phase of AD involves the aggregation of Aβ and Tau, leading to proteostatic stress and homeostatic cellular responses. These responses include autophagy, which is crucial for clearing Aβ and Tau. However, defects in autophagy can contribute to neurodegeneration. The cellular phase of AD is characterized by the interplay between different cell types, including astrocytes, microglia, and oligodendrocytes, which contribute to the disease's progression over decades. The transition from reversible to irreversible cellular responses is a critical phase in AD. Defective clearance mechanisms, such as impaired Aβ and Tau removal, are part of the initial cellular phase. The neurovascular unit, including the blood-brain barrier, plays a significant role in AD, with vascular damage contributing to the disease. The vascular hypothesis suggests that initial vascular damage leads to AD, with implications for Aβ and Tau metabolism and clearance. Neurons and neuronal circuits are central to the cellular phase of AD, with Aβ stress affecting synaptic plasticity and leading to excitatory and inhibitory network imbalances. Astrocytes are crucial in the cellular phase, involved in Aβ catabolism, synaptic support, and modulation of neuronal activity. Reactive astrocytes can contribute to neurodegeneration through impaired clearance and altered signaling. Microglia, the immune cells of the CNS, play a complex role in AD, with both beneficial and harmful effects. TREM2 and CD33 are key genes associated with microglial function and AD risk. Oligodendrocytes, which support myelination and axonal function, are also involved in AD, with myelin breakdown contributing to neurodegeneration. Systems biology approaches highlight the complexity of AD, with multiple pathways and interactions. Single-cell studies provide insights into the heterogeneity of cellular responses in AD, emphasizing the need for cellular resolution in understanding the disease. The cellular phase of AD is a dynamic process involving multiple cell types and pathways, with the transition from reversible to irreversible responses being a critical factor in disease progression.