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ELVIS identifies dark-matter (sub)halos using the six-dimensional halo finder \textsc{rockstar} \citep{Behroozi2013a} and constructs merger trees using the \textsc{consistent-trees} algorithm \citep{Behroozi2013b}.  For each halo that is not a subhalo, we assign a virial mass, $\mvir$, and radius, $\rvir$, using the evolution of the virial relation from \citet{BryanNorman1998}.  We define a ``subhalo'' as a halo whose center is inside $\rvir$ of a host halo.  When a (sub)halo passes within $\rvir$ of a host halo, the (sub)halo becomes its ``satellite'' and experiences ``virial infall''.  For each (sub)halo, we compute the peak mass, $\mpeak$, that it reached along the history of its primary progenitor.