This technical note summarizes and classifies the various approaches to validation of remote measurements of atmospheric state variables, and tries to recommend a clear and unambiguous terminology. The following approaches have been identified: Intercomparison of single profiles for accuracy validation; statistical comparison of matched pairs of measurements with respect to bias determination and precision validation; statistical intercomparison of randomly sampled measurements by two instruments, and comparison of a single measurement to an ensemble of measurements. Applicable statistics are shortly reviewed, and recipes for evaluation of the co-incidence error due to less than perfect co-incidences are presented. A rigorous approach is suggested to quantitatively validate profile measurements when full covariance matrices are unavailable. We distinguish between ''necessary validation'' which is rejection of the null hypothesis that a difference between two measurements is significant, and ''sufficient validation'' which means to provide evidence that the probability that there is a significant difference is definitely small.