Local seeds, which seem adaptive to the local environment, are desirable to plant trees for environmental conservation. Relationships between genetic variation and population fitness are explained based on local adaptation, heterosis, and inbreeding and outbreeding depression, and factors affecting fitness of local seeds are summarized in forest tree populations mainly affected by local adaptation and inbreeding depression. Development of seed transfer guidelines requires knowledge of genetic diversity, seed zoning, and local adaptation, which can be obtained from provenance tests. Provenance test sites with flowering and fruiting trees are valuable research materials although they are potential sources of genetic contamination with native trees.