This essay is concerned with advanced cell culture systems based on the assembly of primary cells within a microfluidic system, thus mimicking the composition, micro-architecture, perfusion, spatiotemporal chemical gradients and function of the smallest functional unit of an organ. It is expected that under these conditions, cells will display in vivo like properties whereas conventional 2D and 3D cell culture systems typically cannot sustain organ-like phenotype and function of cells over extended periods of time. Organ-like cell cultures of different organs such as liver, intestine, kidney, lung, and blood-brain-barrier are anticipated to significantly improve the assessment of toxicity, metabolism, and pharmacokinetics of drug candidates. In addition, mechanistic studies of drug response, cell proliferation, and interaction with cancer cells or pathogens may become feasible in an in vitro environment employing organ-like cultures.