The consequences are assessed of a common set of engineering constraints on the characteristics of fusion reactors that employ deuterium-tritium (D-T), advanced, and exotic fusion fuel cycles. A set of uniform assumptions is made regarding blanket costs, wall loading limits, fusion power density limits, radio-frequency technologies, etc. From these common constraints, the regimes of ion number density, ion kinetic temperature, and plasma stability index β, which lead to attractive fusion reactors, are found. It is demonstrated that if tokamaks are restricted to values of β < 0.05, no fuel cycle other than D-T is compatible with currently accepted engineering constraints. The catalyzed deuterium-deuterium and the D-3He reactions are attractive for values of β > ∼0.20. It is found that the charged particle or “neutron-free” reactions such as ρ-6Li, even if ignitible, are inconsistent with engineering constraints, even at β = 1.0, because of their low reactivity. As expected, the D-T reaction allows the widest range of operating parameters because of its high reactivity. However, it can be used only with difficulty at high values of β because of wall loading limitations. Finally, the limitations imposed by electron cyclotron resonance heating (ECRH) of the plasma are examined. It is found that the cutoff density implied by ECRH (above which radiation is reflected from the plasma) places a serious additional constraint on the accessible operating regime of some advanced fuel fusion reactors.