In recent years, selection programs in meat-type poultry have placed an emphasis on growth rate, muscling, and feed conversion. Although substantial improvements have been made, there have been modification in the morphological structure of the muscle, muscle regeneration mechanisms, and meat quality especially in the breast muscle. In terms of the morphological structure of the breast muscle, muscle fiber diameters have increased, connective tissue spacing has decreased leading to reduced capillary supply, and cellular activity of the satellite cell population has been altered. Many of the necrotic/fibrotic myopathies like Wooden Breast are likely associated with these changes in breast muscle structure and cellular biology. This chapter discusses muscle growth mechanisms and how changes in the cell biology of the breast muscle are related to breast muscle myopathies occurring in meat-type birds.