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The radioactive isotopes of the rare gases provide unique geophysical and mete-orological information concerning the properties and history of our (atmospheric) environment. The extremely low activity levels, however, require very special methods of measurement, experiment design, and data interpretation. Following an examination of the decay characteristics and atmospheric abundances of Kr and Ar radioisotopes, the question of experiment planning and data interpretation in the "extreme Poisson" (few counts) region is considered. A reduced activity plot is offered as an aid for making rapid evaluations and decisions in this region. The concept of an effectively zero background is illustrated by means of atmospheric 37Ar data. Finally, the problem of the blank is examined, and the merits and deficiencies of three alternative approaches are discussed, again with special emphasis on the measurement of 37Ar in Nature.