What are patent maintenance fees, and how much are they?

In the US, maintenance fees are payments due at 3.5, 7.5 and 11.5 years after a utility patent grants, required to keep it in force for the full 20-year term, with each payment larger than the last; US design and plant patents don't require maintenance fees at all. Most other countries, including the UK and Germany, instead charge annual renewal fees that rise each year, often starting while the application is still pending.