Man’s impact on forest tree populations worldwide and in Central Europe in particular has affected their genetic variation in the past and will continue to do so. The parental pool of tree populations is becoming smaller, causing loss of genetic information. Some tree species have become rare or fragmented into reproductively isolated populations. Populations of other species were planted from reproductive material that was shifted over ecological gradients or introgressed from neighboring populations of formerly allopatric species. The amount of genetic variation within populations and the differentiation between them is characterised by genetic parameters that are indispensable in designing measures for the conservation of gene resources of sufficient size in situ or ex situ. The genetic consequences of widely used forest operations, including the breeding of trees for desired traits, is analysed. Finally, the genetic implications of legislation are considered. Regrettably, the conservation of genetic resources currently receives little public attention, even though it provides the basis of sustainable species management.