The motion of excitons in organic semiconductors is critical to the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). The authors quantify the contributions of energy transfer and diffusion to the migration of excitons through the thin organic film in an OLED. Their results emphasize that an exciton's $e\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ and $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ diffusion lengths can differ strongly, under some conditions. This thorough, comprehensive study will inform rational device design, particularly of white OLEDs for solid-state lighting.