The IUTAM Symposium on Motile Cells in Complex Environments (MCCE2018) will bring together experts in the complementary fields of physics, applied mathematics, chemistry, biology, life sciences, and engineering to discuss multidisciplinary theoretical, numerical and experimental approaches to predict the behaviour of active complex fluid systems characterized by the presence of motile living cells. Cell motility is a multi-faceted interdisciplinary challenge for a wide community of scientists, with applications ranging from medical to bio-technological and environmental issues. Cells often grow and move within complex fluid environments: Examples include motile phytoplankton cells giving rise to bio-convective patterns in turbulent flows, but also suspensions of swimming bacteria that can form biofilms under adverse conditions and swarm to colonize solid surfaces. Depending on the targeted biological system, such environments may be characterized both by high and low Reynolds numbers, thus, involving a broad spectrum of spatial and temporal dynamics that are not easy to model.