Breast cancer xenograft imaged in vivo by volumetric multispectral optoacoustic tomography (vMSOT). Fine vasculature patterns in the superficial epidermal layer are visible along with the larger feeding vessels and the necrotic core. The unique spectral differentiation capacity of vMSOT grants label-free measures of the blood oxygenation at high resolution across the entire tumor volume, while hypoxic fraction and cyclic hypoxia can further be dynamically quantified during oxygen challenge. For details, see article by Ron and colleagues on page 4767.