The oocytes of the South African clawed frog X. laevis are widely used for the expression of heterologous proteins. The functional characterization of membrane proteins in particular has significantly profited from the use of this expression system. Heterologous cRNA can easily be injected and protein expression and function be studied with several techniques. This review will give a short overview into the variety of methods applicable. They span from different electrophysiological methods such as two electrode voltage clamp, patch clamp and ion-selective electrodes over cytochemistry to protein biochemistry. In spite of the wide usage of Xenopus oocytes, caution should be taken interpreting the results of protein expression. Heterologous proteins may either interact with endogenous proteins, the background of endogenous protein function may be relatively high, or altered protein behaviour may occur due to differences of the ambient temperature or altered cellular environment.