In order to make genetic progress for an economically important trait, the trait (or a closely related trait) must be measurable, heritable, and have sufficient additive genetic variation. Genetic parameters must be estimated to determine the amount of additive genetic variation and heritability for use in a genetic evaluation and selection program. Superior individuals for the trait of interest must be identified and retained for breeding purposes to effectively implement a successful breeding program. Finally, the selected individuals must be mated in a way to produce the maximum response while limiting any negative impact selection for the trait of interest may have on other economically important correlated traits. In this thesis, four projects were conducted to investigate intramuscular fat percentage, an important pork quality trait, its measurement in live swine, estimation of genetic parameters, and determination of its relationship with carcass, meat, and eating quality traits, and implementation of this trait into a selection experiment to improve the trait.;In the first project, purebred Durocs (n = 207) were used to develop a model using real-time ultrasound technology to predict loin intramuscular fat percentage of the longissimus muscle in live pigs. In the second project, data from two national progeny testing programs were used to compare the relationships of intramuscular fat percentage of the longissimus predicted using real-time ultrasound and chemical intramuscular fat percentage with meat quality traits in pigs. In the third project, data from a selection project designed to increase intramuscular fat percentage (IMF) in a Duroc swine population were utilized to compare three different models used to estimate breeding values for IMF. In the final study, data from randomly mated and selected Duroc pigs (n = 589) were used to determine the genetic and phenotypic relationships between individual subcutaneous backfat layers and intramuscular fat percentage of the loin. Results of the first project showed real-time ultrasound image analysis can be used to predict intramuscular fat percentage in live swine.;Results from the second project demonstrated that selection for intramuscular fat percentage estimated from chemical analysis or by real-time ultrasound should yield similar genetic changes in other meat quality traits in pigs. Results from the third project indicated that selection based on a combination of ultrasonically predicted IMF and sib carcass IMF produced the greatest selection differentials and should lead to the greatest genetic change when compared to selection based on ultrasound estimates of IMF alone. Results from the final project demonstrated that individual backfat layers are highly heritable and of similar magnitude as total backfat, and have similar genetic correlations with IMF. The outer or inner backfat layers could be implemented into a multiple-trait genetic evaluation, instead of total backfat, to improve IMF.;Overall, the results from the four projects contained in this thesis indicate measuring intramuscular fat percentage of the porcine longissimus can be accurately performed utilizing real-time ultrasound, the genetic parameters and relationships with other meat and eating quality traits for IMF predicted from real-time ultrasound are similar to those of IMF estimated from the carcass, and substantial genetic progress can be made when implementing this technology into a selection program. In addition, inclusion of a single backfat layer into the genetic evaluation for intramuscular fat percentage could increase in the amount of genetic progress realized through selection.