Long-lived and large-scale anticyclonic Aleutian eddies (AEs), detaching from the Alaskan Stream to the west of 180∘ and propagating southwestward or almost zonally in the Western Subarctic Gyre, have been investigated during 1993–2020 through Lagrangian analysis of altimetry data and Argo float profiles. All such AEs in the altimetry era have been identified, tracked and documented providing the first systematic census of this class of eddies. Inspecting daily computed Lagrangian maps of the origin of water masses and of the distance travelled by fluid particles, we followed as the AEs moved from the formation area toward the Kamchatka Peninsula. The Lagrangian diagnostics allowed us to document the main events in the lifecycle of each AE including interaction with bottom topography, deformation, splitting, merge, erosion and eventual decay. Just after formation, the AEs stagnate for a while over the Aleutian Trench before isolating from the Stream. They experienced strong deformation when crossing the Detroit Rise, and some have been found to split when passing the Rise. Kinematics and vertical structure of a few previously sampled AEs have been considered in detail using the altimetry-based Lagrangian maps and Argo float observation. The observations reveal typical subarctic vertical structure with the specific warm and saline mesothermal water in the intermediate layer. The Aleutian eddies transport warm and saline water under a cold upper layer that eventually contributes to the mesothermal layer off Kamchatka. [ABSTRACT FROM AUTHOR]