The tumor microenvironment (TME) plays a key role for tumor biology. Composition of the TME correlates with overall survival and governs therapy response. Non invasive assessment of the TME has been notoriously difficult. We have designed an imaging strategy to non invasively visualize innate immune cell dynamics in the TME by correlated MRI and multiphoton microscopy. This approach allowed us to visualize the single steps of nanoparticle uptake by blood born monocytes that give rise to tumor macrophages in an experimental glioma model. We further found that nanoparticle uptake also occurs via the disrupted blood-brain barrier into the brain parenchyma where NP are taken up by tumor associated microglia.