I will describe the operation principles of optical clocks and intrinsic differences between systems that rely on the laser interrogation of a single ion or neutral atoms. Illustrated by selected examples, I will discuss the present performance of optical clocks. Here, the accuracy of the frequency standards plays an important role, i.e., how well the system reproduces the unperturbed atomic transition frequency. Indeed, the interaction between atom and the environmental thermal radiation field can lead to substantial frequency shifts that have to be accounted for. I will discuss different approaches to determine and correct these frequency offsets and present experimental strategies to control or minimize the shifts.