  In this article we study congruences of lines in $\mathbb{P}^n$, and in
particular of order one. After giving general results, we obtain a complete
classification in the case of $\mathbb{P}^4$ in which the fundamental surface
$F$ is in fact a variety-i.e. it is integral-and the congruence is the
irreducible set of the trisecant lines of $F$.
