The APZ questionnaire was developed in order to explore hypotheses on ASCs. First -- in a series of 11 experiments using different induction methods on N = 393 healthy subjects -- the hypothesis was tested that ASCs have major dimensions in common irrespective of the mode of their induction. In the International Study on Altered States of Consciousness (ISASC) the external validity of the experimental results was assessed. The ISASC was carried out on a total of N = 1133 subjects in six countries. The main results of the experimental studies were corroborated in the field studies. The results can be summarized as follows: the common denominator of ASCs is described by three oblique dimensions, designated as "Oceanic Boundlessness (OSE)", "Dread of Ego Dissolution (AIA)" and "Visionary Restructuralization (VUS)". The reliability and validity of the scales are satisfactory. Tested versions of the APZ scales are available in English (UK, USA), German, Italian and Portuguese. Psychometrically as yet untested versions exist in Dutch, Finnish, French, Greek, Spanish and Russian. The APZ questionnaire has become the international standard for the assessment of ASCs, thus helping to integrate research. A psychometrically improved version exists in German (OAV questionnaire). The BETA questionnaire, which measures the dimensions "Vigilance Reduction (VIR)" and "Auditive Alteration (AVE)" is also available in German. These dimensions are most likely etiology-dependent.