**1. Introduction**

 All-*trans*-retinoic acid (atRA) is the transcriptionally active derivative of vitamin A. atRA is an essential signaling molecule for developmental processes of numerous organ systems including those of the brain, limbs, lungs, pancreas, heart, and eye in many organisms from fish to humans [1, 2]. With the advent of increasingly sensitive methods to measure endogenous concentrations of atRA, data is accumulating to suggest that atRA is also important in the growth and maintenance of a number of organ systems during postnatal and adult life [3–7]. Within the postnatal eye, atRA has been detected in the retina, where its synthesis was shown to be mediated by oxidation of the chromophore all-*trans*-retinaldehyde, released from bleached rhodopsin in the photoreceptor outer segments following exposure to light [8]. More recently, the choroid (the highly vascular layer between the retina and the sclera) has been shown to synthesize and accumulate high levels of atRA [9, 10]. A number of studies in several animal models suggest that choroidal atRA may be an important molecular signal for the control of postnatal ocular growth [9, 11–13]. We and others have demonstrated that in response to visual stimuli, ocular atRA synthesis is regulated exclusively via choroidal expression of the atRA synthe sizing enzyme, retinaldehyde dehydrogenase 2 (RALDH2) by a unique population of cells [10, 14]. Furthermore, choroidally derived retinoic acid is transported to the sclera (the outer connective tissue shell of the eye) by apolipoprotein A-1 (ApoA-1) which functions as a specific extracellular atRA-binding and transport protein in the eye [15, 16]. Once delivered to the sclera, we speculate that atRA regulates the transcription of many genes in the sclera to effect changes in scleral extracellular matrix remodeling, ocular size, and refraction.

This chapter therefore focuses primarily on the potential role of atRA on the control of postnatal growth of the eye, and implications for the development of new therapies for the control of myopia in children.
