Why do we need to build quantitative mechanistic models of biological systems? What are virtual tissues? Sample simulations: vascular tumors, polycystic kidney disease, liver toxicity, age-related macular degeneration, in host models of viral infection and immune response. Running CompuCell3D on nanoHUB servers (https://nanohub.org/resources/cc3dbase4x). CompuCell3D tools, the Twedit++ model editor and the Player model execution tool. Running a 3D Angiogenesis simulation. Creating a simple simulation of cell growth and division using Twedit++. Resources for CompuCell3D (https://compucell3d.org/Manuals).