Brain organoids, also called “mini brains”, represent a powerful tool for studying human neurological diseases, particularly those that affect brain growth and structure. However, many diseases manifest with clear evidence of physiological and network abnormality in the absence of anatomical changes, raising the question of whether organoids possess sufficient neural network complexity to model these conditions. Here, we explore the network-level functions of brain organoids that reveal the existence of complex network dynamics reminiscent of intact brain preparations. We demonstrate highly abnormal and epileptic-like activity in organoids derived from induced pluripotent stem cells from individuals with Rett syndrome. We also rescue key physiological activities with an unconventional neuroregulatory drug. Together, these findings provide an essential foundation for the utilization of brain organoids to study intact and disordered human brain network formation and illustrate their utility in therapeutic discovery.