Infrastructure systems for energy, water, transport, information etc. are large scale socio-technical systems that are critical for achieving a sustainable world. They were not created at the current global scale at once, but have slowly evolved from simple local systems, through many social and technical decisions. If we are to understand them and manage them sustainably, we need to capture their full diversity and adaptivity in models that respect Ashby's law of requisite variety. Models of evolving complex systems must themselves be evolving complex systems that can not be created from scratch but must be grown from simple to complex. This paper presents a socio-technical evolutionary modeling process for creating evolving, complex agent based models for understanding the evolution of large scale socio-technical systems such as infrastructures. It involves the continuous co-evolution and improvement of a social process for model specification, the technical design of a modular simulation engine, the encoding of formalized knowledge and collection of relevant facts. In the paper we introduce the process design, the requirements for guiding the evolution of the modeling process and illustrate the process for Agent Based Model development by showing a series of ever more complex models.