An extended scalar sector model posits that there are more scalars interacting with us than just the Higgs doublet of the Standard Model. These extra scalars can significantly alter the mechanism of electroweak symmetry breaking relative to that of the Standard Model.
Extended scalar sectors are one of the strongholds of viable and motivated electroweak-scale new physics, and one of the key classes of models probed by future precision measurements of the electroweak sector, as well as gravitational wave signals of the electroweak phase transition in the early universe.
These extra scalar states are not "heavy" or "decoupling", and they are arguably the last place that one should use an effective field theory (EFT) of local operators to describe the deviations from Standard Model predictions that the scalars produce. I will describe when effective field theory works, when it doesn't, and some EFT tricks to elucidate the universal behaviours in extended scalar sectors. I will argue that, if additional scalars are significantly altering the mechanism of electroweak symmetry breaking, we are practically guaranteed to see them at the next generation of experiments.