The aim of this study was to show the practical steps to be followed in the adaptation of instruments for measuring psychological constructs across cultures. For this purpose, the adaptation process of the NEO PI-R from Spanish into Basque is thoroughly described. In order to avoid the sources of error that researchers could encounter during the adaptation process, established guidelines are followed. In the stage which focuses on the empirical validation of the instrument to be used, four studies are described that examine the following aspects: dimensionality, convergent and differential validity, internal consistency of each dimension, and the possible differential item functioning with regard to gender and life stage (youth or adulthood). In the light of the results obtained, we conclude that the Basque version of NEO PI-R guarantees an adequate assessment of personality from the Big Five viewpoint. These studies illustrate how to obtain evidence about the degree of validity of the adapted version of a test, which is the key feature of any adaptation process.