  In this expository note, I present some basic geometric and twistor theoretic
facts about de Sitter space leading up to a discussion of Penrose's quasi-local
mass construction for linear gravity theory in the de Sitter background. The
corresponding results for Minkowski space and anti--de Sitter space are
familiar and can be found in \cite{PR} or \cite{HT} for the former and
\cite{KT} for the latter. Some of the formulas given here are also quite
familiar, but some are thought to be new and it is convenient to have these
facts collected in one place.
