Different tests can be used to provide valuable information about the function of the peritoneal membrane. The data obtained can be useful for tailoring dialysis adequacy, for the analysis of clinical problems such as ultrafiltration failure or to predict the development of more severe peritoneal membrane derangement. The most widely used peritoneal function test is the peritoneal equilibration test (PET), developed and described by Twardowski in 1987. PET is performed using a 2.27% glucose solution, and it lasts 4 hours. It measures peritoneal solute transport and ultrafiltration (UF), and it gives the possibility to categorize patients (high, high-average, low-average and low transporters). However, a PET with 3.86% glucose provides better information on UF, on the phenomenon of sodium sieving and an assessment of free water transport. Two recently developed tests (Mini-PET, Double Mini-PET) are promising tools to assess free water transport and the osmotic conductance to glucose. It is possible to integrate the 3.86%-PET with the new tests to obtain a complete PET: the Uni-PET (all these tests in 1 PET). The new insights into peritoneal function need a new standardization of the PET. It would be worth having a machine (PET machine) which performs the PET automatically to avoid possible mistakes during the performance of the manual PET and to allow an universal standardization of the test.