I will give a brief overview of the thermodynamic uncertainty relation and discuss its application to steady-state heat engines. It shows that the constancy of power plays a crucial role in the trade-off between power and efficiency: Getting close to Carnot efficiency at finite output power is only possible at the cost of diverging power fluctuations. Then, I will show examples for isothermal engines driven by a bath of active matter. Here, the overall efficiency is generally low even for highly optimised designs of engines, but collective effects can lead to an enhancement of power and efficiency.