You didn't mention what your end goal was by filtering by certifications, so I'll give you an example. Here's a query that will only include movies with an MPAA (US) certification of PG-13:

https://api.themoviedb.org/3/discover/movie?api_key=###&certification=PG-13&certification_country=US

You can also use certification.gte and certification.lte to build more dynamic queries.

Travis, isn't there a possibility to add this to tvshows too?

@neufke said:

Travis, isn't there a possibility to add this to tvshows too?

and in reply top my own question:

https://api.themoviedb.org/3/discover/tv?api_key=###&certification=TV-MA&certification_country=US

is working great, thanks!