On the Complexity of the Planar Slope Number Problem Vol. 21, no. 2, pp. 183-193, 2017. Concise paper. Abstract The planar slope number of a planar graph $G$ is defined as the minimum number of slopes that is required for a crossing-free straight-line drawing of $G$. We show that determining the planar slope number is hard in the existential theory of the reals. We discuss consequences for drawings that minimize the planar slope number. Submitted: October 2016. Reviewed: December 2016. Revised: January 2017. Accepted: January 2017. Final: January 2017. Published: January 2017. Communicated by Giuseppe Liotta article (PDF) BibTeX