This report summarizes the talks, breakout sessions, and discussions at the Dagstuhl Seminar 17191 on "Theory of Randomized Optimization Heuristics", held during the week from May 08 until May 12, 2017, in Schloss Dagstuhl -- Leibniz Center for Informatics. The meeting is the successor of the "Theory of Evolutionary Algorithm" seminar series, where the change in the title reflects the development of the research field toward a broader range of heuristics. The seminar has hosted 40 researchers from 15 countries. Topics that have been intensively discussed at the seminar include population-based heuristics, constrained optimization, non-static parameter choices as well as connections to research in machine learning.