Insulin-like growth factor I (IGF-I) plays a critical role in the growth and development of many tissues in the body. It is a key regulator of skeletal muscle development, and continues to enhance the ability for muscle to grow and undergo repair throughout life. Although the focus of research has been on the molecular actions and physiological impact of IGF-I, there has also been a growing undercurrent of studies geared toward the characterization of additional potentially active peptides produced by the igf1 gene. Alternative splicing of the gene results in multiple isoforms that retain the identical sequence for mature IGF-I, but also give rise to divergent C-terminal peptides. The peptides might modulate the actions, stability, or bioavailability of IGF-I, or they might have independent activity. These possibilities have gained the attention of the skeletal muscle field, where novel actions of IGF-I could have significant impact on muscle mass, strength, and repair.