Large ground-based telescopes equipped with adaptive optics (AO) systems have ushered, in a new era of highresolution infrared photometry and astrometry. Relative astrometric accuracies of <0.2 mas have already been demonstrated, from infrared images with spatial resolutions of 55-95 mas resolution over 10-20" fields of view. Relative photometric accuracies of 3% and absolute photometric accuracies of 5%-20% are also possible. I will review improvements and current limitations in astrometry and photometry with adaptive optics of crowded stellar fields. These capabilities enable experiments such as measuring orbits for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole and its environment, and identifying individual stars in young star clusters, which can be used test the universality of the initial mass function.