For closed-loop control as well as on-board-diagnosis in combustion engines, there is increasing interest for online emission measurement. This paper shows a comparative evaluation of online nitrogen oxides (NOx) sensors for production Diesel engines. Two totally different types of sensors are compared: The first is a physical solid-state sensor and the second is a virtual sensor where NOx values are not measured but calculated by a mathematical model. Specifications of the sensors are presented, advantages and disadvantages are analyzed and ways of combinations are mentioned.