Complex Systems Summer School (CSSS) offers an intensive four-week introduction to complex behavior in mathematical, physical, living, and social systems. CSSS brings together graduate students, postdoctoral fellows, and professionals to transcend disciplinary boundaries, take intellectual risks, and ask big questions about complex systems. The residential program comprises a series of lectures and workshops devoted to theory and tools, applications-focused seminars, and discussions with faculty and fellow participants. CSSS participants put what they learn from these didactic sessions into practice through group research projects, conducted throughout the program and often extending into manuscripts and longer-term collaborations. CSSS provides an unparalleled opportunity for early-career researchers to expand their professional networks, produce a novel research product, and gain valuable experience working in transdisciplinary teams.