In 2010, the Illinois Supreme Court invalidated certain statutory caps on non-economic damages in medical cases because they “unduly” infringed “upon the inherent power of the judiciary.” Such judicial authority originated within the separation of powers clause of the Illinois Constitution. The caps were deemed to “encroach” on the judiciary’s “sphere of authority” because they impeded “the courts in the performance of their function.” Elsewhere, American state statutory damage caps have also been challenged on state constitutional separation of powers grounds. These challenges include settings where the caps operate for non-medical cases and where the limits extend beyond non-economic damages. Given state constitutional diversity, are there core separation of powers principles guiding all American state statutory damage caps? If so, do they apply similarly to all forms of damage caps? With or without such core principles, are there other doctrines, not involving individual rights, that better speak to damage caps? The paper first explores the Illinois precedents on damage caps and separation of powers. Then it examines other state precedents, where it finds the controversies usually involve allocations of civil procedure lawmaking powers. It observes that caps on “statutory causes of action” or during “special proceedings” are often treated differently, as are caps on punitive damages and caps that operate in alternative public forums like administrative agencies. The paper concludes that separation of powers analyses should usually be replaced in damage cap cases with judicial rulemaking analyses. It finds no core principles within state constitutional separation of powers provisions that implicate damage caps. Interstate differences in constitutional allocations of procedural rulemaking authority (and, at times, of justiciable matters) should be recognized more often. This last observation has implications beyond damage caps. Judicial/legislative tensions elsewhere, as with evidence privileges, should also be explored with judicial rulemaking analyses.