Self-sensing actuation in piezoelectric actuators has been a growing research topic since 1992, with two main approaches having been identified: either by evaluating the dielectric displacement field within the piezoelectric through a measurement bridge, either by measuring the amount of accumulated charges. An overview of the principle of operation for each approach is laid out, followed by key research topics that have improved each approach over the years. The displacement and force quasi-static estimation accuracy for each approach is then discussed and compared based on referenced examples in published litterature. Finally, it is observed that the charge based self-sensing approach has been favoured in recent research, and has been showing more promising results in terms of displacement estimation.