The use of value-added modeling (VAM) in school accountability is expanding, but deciding how to embrace VAM is difficult. Various experts say it's too unreliable, causes more harm than good, and has a big margin for error. Others assert VAM is imperfect but useful, and provides valuable feedback. A closer look at the models, and their use, legitimizes all of these statements. Teachers do have measurable effects on students and any effort to create a quality teaching force should include a system that holds them accountable. VAM has the potential to improve such an accountability system by isolating teacher effects.