J. A. Lewis and co-workers report on page 3124 a new bio-printing method for fabricating 3D tissue constructs replete with vasculature, multiple types of cells, and extracellular matrix. These intricate, heterogeneous structures are created by precisely co-printing multiple materials, known as bio-inks, in three dimensions. These 3D microengineered environments open new avenues for drug screening and tissue engineering. Cover image by Helena Sue Martin, David Kolesky, and Jennifer Lewis (Harvard University).