<p>The extracellular matrix (ECM) constitutes the bulk between cells, establishes barriers that separate tissue compartments, and defines the form and pliability of organs. The ECM is an integral part of the cellular microenvironment, conveying a multitude of biochemical (and mechanical) signals to associated cells, for example, by binding cytokines and chemotactic factors. All cells are in contact with the ECM, including immune cells in lymphoid organs and when they extravasate from blood vessels, as well as tissue-resident myeloid cells in close contact with the ECM of blood vessels. The past decade has seen technical advances in proteomics and transcriptomics that have provided large volumes of data on the diversity of the ECM and of the immune system. It is important to assess this rapidly growing body of data together with knowledge of the in vivo situation, particularly from more than a decade of intravital imaging studies. This Review summarizes both biochemical and imaging data that are relevant to leukocyte extravasation across the ECM — in particular, the basement membrane — to enter sites of inflammation, as well as the changes to the ECM that are associated with chronic inflammation, including tumour sites. It also discusses how this information may be exploited for the development of novel immunotherapies.</p>