Parties engaged in multidistrict litigation (MDL) face a crucial decision: which case or cases should be tried first? For both plaintiffs and defendants, bellwethers — the first trial or trials from the similar cases making up the MDL — can determine how the rest of the cases proceed. One current headlining case — the prescription opioid MDL pending before Judge Polster in federal district court in Ohio — shows both how hard it is to select bellwethers and why bellwethers matter.