Special issue on Selected papers from the Twenty-fifth International Symposium on Graph Drawing and Network Visualization, GD 2017 The Effect of Planarization on Width Vol. 22, no. 3, pp. 461-481, 2018. Regular paper. Abstract We study the effects on graph width parameters of planarization, the construction of a planar diagram from a non-planar graph drawing by replacing each crossing with a new vertex. We show that for treewidth, pathwidth, branchwidth, clique-width, and tree-depth there exists a family of $n$-vertex graphs with bounded parameter value, all of whose planarizations have parameter value $\Omega(n)$. However, for bandwidth, cutwidth, and carving width, every graph with bounded parameter value has a planarization of linear size whose parameter value remains bounded. The same is true for the treewidth, pathwidth, and branchwidth of graphs of bounded degree. To show our lower bounds on the width of planarizations, we prove that arrangements of curves with many crossing pairs of curves must generate planar graphs of high width. Submitted: October 2017. Reviewed: March 2018. Revised: April 2018. Accepted: April 2018. Final: May 2018. Published: September 2018. Communicated by Fabrizio Frati and Kwan-Liu Ma article (PDF) BibTeX