This working group, Integrating Ecological Theories From a First Principles Perspective, seeks to synthesize and integrate ecological theories and models. This will be the fifth meeting of a formal international academic collaboration network, the Network for Ecological Theory Integration (NETI), whose founding members are a group of scientists from Chile, Uruguay, USA, Australia, and Europe, many of whom are associated with the Santa Fe Institute. The first NETI meeting was held in Chile in 2014 and the most recent meeting was held at SFI in 2018. NETI´s aim is to promote integration of mathematical theories in ecology, with a particular focus on metabolic theory, neutral theory, maximum entropy theory, and network theory. We are interested in efficient theories, i.e., theories which are quantitative, based on first principles, and prediction rich. The scientists involved with NETI, a combination of ecologists, physicists, and mathematicians, seek to balance the current emphasis upon big data by emphasizing the need for big theory in biology and ecology in particular. Such synthesis and integration of theories provides an important step forward in our understanding of the structure and dynamics of ecological complexity, which is needed to provide more informed and useful answers to the pressing problems of the Anthropocene.