A self-consistent integral equation is formulated and solved iteratively which determines the steady-state lasing modes of open multi-mode lasers. These modes are naturally decomposed in terms of frequency dependent biorthogonal modes of a linear wave equation and not in terms of resonances of the cold cavity. A one-dimensional cavity laser is analyzed and the lasing mode is found to have non-trivial spatial structure even in the single-mode limit. In the multi-mode regime spatial hole-burning and mode competition is treated exactly. The formalism generalizes to complex, chaotic and random laser media.The steady-state electric field within and outside of a single or multi-mode laser arises as a solution of the non-linear coupled matter-field equations, the simplest of which are the two-level Maxwell-Bloch equations treated below. While the basic equations involved have been known for many years, and many aspects of their temporal dynamics have been studied [1], relatively little progress has been made in understanding the spatial structure of the non-linear electric field, particularly in the case of multi-mode solutions for which spatial holeburning and other non-linear effects are critical. It is natural to attempt to understand the non-linear solutions in terms of solutions of a linear wave equation. The two standard choices are either the hermitian solutions of a perfectly reflecting (closed) passive laser cavity [2], or the non-hermitian non-orthogonal resonances of the open passive cavity [3,4]). In fact the intuitive picture of a lasing mode is that it arises when one of the resonances of the passive cavity is "pulled" up to the real axis by adding gain to the resonator. Often comparison of the numerically generated lasing modes with calculated linear resonances do show strong similarities in spatial structure, providing useful interpretation of lasing modes [5,6], although not a predictive theory. However with the current interest in complex laser cavities based on wave-chaotic shapes [7,8], photonic bandgap media [9,10] or random media [11,12] it is important to have a quantitative and predictive theory of the lasing states, as the numerical simulations required to solve the timedependent Maxwell-Bloch equations are time-consuming and not easy to interpret.In recent work we have formulated a theory of steadystate multi-mode lasing which addresses these concerns [13]. The theory implies that the natural linear basis for decomposing lasing solutions is the dual set of biorthogonal states corresponding to constant outgoing and incoming Poynting vector at infinity at the lasing frequencies (referred to as "constant flux" (CF) states). Our theory shows that even in conventional lasers it is incorrect to regard the lasing modes as corresponding to a single resonance of the passive cavity and that multiple spatial frequencies occur even when there is a single lasing frequency close to the frequency of a single passive cavity resonance. These multiple spatial frequencies arise because several CF states contribu...