The extracellular matrix (ECM) is a network woven out of more than 1300 different proteins, of which ≈300 are structural. Their presence, distribution, and abundance change between and within tissues. It is also increasingly clear that the ECM is remodeled in disease-specific patterns. The interactions between organ- or disease-specific ECM and resident cells are a subject of intense research and engineering. Precisely mapping the three-dimensional ECM structure across tissues and diseases is therefore a fundamental task. Here, we discuss in situ decellularization of tissues (ISDoT) as an essential tool to map the ECM supporting primary and metastatic tumors in experimental mice.