Product quantity and quality have been substantially improved in many previously unselected forest tree populations by one generation of selection. Estimated and realized heritabilities have been appreciable for numerous traits and may well increase under more intensive culture. However, environmental variations are higher than for most agricultural crops and more diverse sites for afforestation are expected in the future. Also, unlike most agricultural crops, trees take several years to mature both sexually and economically. Therefore the changes in ecological and economic desiderata per breeding cycle are large and demand that breeding populations be adaptable for variable futures. These factors require that screening for yield be highly efficient in use of time and materials and hence necessitate the use of multiple trait and indirect selection, use of information on relatives, and tests performed in a wide range of environments. Screening procedures thus commonly involve use of progeny tests with mating and environmental designs which efficiently estimate entry means, genetic and environmental variance-covariance matrices and genotype-environment interactions, and which often also provide materials for recurrent selection. Several component units of yield are assessed at various ages to estimate juvenile-mature performance correlations and the type of genetic control of each. These include anatomical, physiological, morphological and chemical traits which affect either ecological or economic utility.