Bilateral Teleoperation is a key technology to allow humans to interact with remote environments by providing the operator with haptic feedback. Haptic feedback from the one hand improves human perception and therefore the quality of the human-robot interaction, on the other hand it can tamper with the stability of the system when the communication between the master side (where the operator is) and the slave side (where the remote robot interacts with the environment) is not instantaneous but affected by delay and packet drops. In the last 40 years many algorithms have been developed to guarantee the stability of haptic teleoperation in the presence of time delay, many of them based on passivity theory. In thispaper we review and compare a few algorithms that are representative of the tools in the frequency or in the time domains that have been used to develop a safe and transparent physical human-robot interaction with unknown environments.