Abstract Computer modeling of components and systems has revolutionized engineering design. Yet, most computer-aided design (CAD) approaches follow the strategy employed in traditional practice: an initial model, created with empirical or approximate rules, is tested (analyzed) and modified (optimized) until it meets specifications. The key concept in this CAD approach is thus the efficient combination of numerical analysis with an optimization strategy. In this paper we explore a new approach to electromagnetic design of waveguide filters and aim at creating an alternative design methodology. The main idea consists in exploiting the intrinsic optimization capability of electromagnetic fields to minimize energy storage and losses, not by means of a variational formulation, but by exploiting the topology of three-dimensional standing wave pattern created in space. This new field synthesis approach relies on time-domain modeling of electromagnetic fields and employs the Transmission-Line Matrix (TLM) modeling method. After a theoretical introduction tothe synthesis methodology we describe a step-by-step synthesis of an iris-coupled rectangular waveguide bandpass filter and verify the result by performing a rigorous field analysis of the synthesized topology.