This chapter describes corrections that have to be applied to measured membrane potentials in patch clamp experiments. Some of them [Eqs. (1)-(3)] are required regardless of the nature of the reference electrode (in the Ringer's solution bath) whenever the pipette-filling solution is different from the bath solution. They represent the liquid junction potentials that are present at the pipette tip before patch formation. In addition, corrections have to be applied when the bath solution is being changed during a measurement (i.e., after seal formation). In that case the following rules apply. (1) The new solution should never get into contact with the bare silver/silver chloride wire of the reference electrode. This requirement is best met by using a salt bridge. (2) The "best" salt bridge is a 3 M KCl bridge with an abrupt KCl-bath fluid boundary at its tip (see above). This bridge does not require any additional potential corrections, but it may lead to KCl poisoning of the bath or become contaminated by solutions used previously. (3) Local solution changes (microperfusion by puffer pipette, U tool or sewer pipe arrangements) as well as recessed KCl bridges require additional corrections, which (together with the simple liquid junction potential correction) are approximately given by Eqs. (6)-(8). It should be stressed that all equations given here represent approximate corrections, since liquid junction potentials are thermodynamically ill-defined. This is particularly relevant for Eqs. (6) and (7) where the sum of two liquid junction potentials appears.