The streaming war between Disney and Netflix has captured the attention of investors, the media and viewers. But a lower-profile yet equally high-stakes battle is underway in the streaming video market—between entertainment companies and a new generation of gatekeepers. The players range from big tech giants like Amazon to smaller firms like Roku.

Roku, a streaming device maker with a market capitalization of $16 billion, isn’t one of the tech majors. But it has nearly 40% of the market for streaming devices, making it the leading way many households stream video onto their TV sets. Entertainment executives say that Roku frequently tries to flex that power to extract more money from entertainment companies whose apps are carried on Roku devices—with mixed results so far.