The selection of populations as genetic resources continues to be one of the problematic features in forest gene conservation. Different approaches which lead to the detection of in situ resources are critically reviewed. In particular the detection based on an ecological approach, on fitness relevant traits, and on different types of genetic markers are described in greater detail. Factors not related to genetics, to be taken into account when selecting appropriate populations, are also mentioned. It is concluded that a selection that is exclusively based on genetic markers is not appropriate. Different marker types commonly result in different findings and their related genetic geographic pattern usually does not reflect genetic differentiation of fitness relevant traits. Since marker variation can be useful in individual cases, results derived from field studies and from investigations based on different marker types are available for the selection of in situ resources.