Life is very complicated. Recently, a group of review papers on the complexity of various biological behaviors were published by Cell to celebrate its 40th anniversary. The developmental process must be one of the most complex events in multi-cellular organisms. JMCB has published several research papers in the previous Collection ‘Understanding and Manipulating Developmental Complexity’ (Wu, 2013). This issue publishes more articles focusing on the molecular mechanisms that regulate animal developmental processes, which should provide evidences for a better understanding of the developmental complexity.