  This paper is devoted to introduce and investigate the notion of monotone
sets in Hadamard spaces. First, flat Hadamard spaces are introduced and
investigated. It is shown that an Hadamard space $X$ is flat if and only if
$X\times X^\medlozenge$ has $\mathcal{F}_l$-property, where $X^\medlozenge$ is
the linear dual of $X$. Moreover, monotone and maximal monotone sets are
introduced and also monotonicity from polarity point of view is considered.
Some characterizations of (maximal) monotone sets, specially based on polarity,
are given. Finally, it is proved that any maximal monotone set is sequentially
$bw\times${$\|\cdot\|_\loz$}-closed in $X\times X^\medlozenge$.
