With the common use of highly sensitive electronic pressure recorders and the widespread use of reservoir characterization by type-curve pressure-derivative techniques, preprocessing data has now become an important prerequisite to pressure-transient analysis. In almost all cases during a well test, the electronic pressure recorders measure a multitude of effects other than the intended reservoir pressure transients. For example, multiphase, geotidal, microseismic, changing liquid levels, recorder drift, recorder plugging, etc. can mask the reservoir transient. If not properly identified, these effects could easily be misinterpreted as reservoir characteristics, such as pressure depletion, boundaries, or dual porosity. The preprocessing of the digital data, starting with the comparison of pressures from multiple recorders and ending with data filtration to remove the noise associated with the signal, is a significant component of well-test interpretation and is a prerequisite to pressure-transient analysis. From experience with thousands of well tests, we have chosen several examples to illustrate the need for preprocessing the data. The observed pressures must be corrected to remove the nonreservoir effects before pressure-transient analysis is attempted. Often, diagnosis of the observed data can only be done with the help of information other than thr pressure trace.