A collision with a tractor-trailer is not a bigger car accident. It is a different case, governed by a body of federal safety regulation that has no analogue in ordinary driving, litigated against a corporate defendant with a rapid response team, and turning on electronic evidence that begins overwriting itself within days. This article explains how commercial vehicle cases are actually built: the regulations that create the duties, the documents that prove violations, the corporate theories that reach past the driver, and the preservation letter that has to go out in the first week. It covers hours of service and electronic logging, driver qualification files, maintenance records, drug and alcohol testing, the MCS-90 endorsement that pays when nothing else will, and the broker and shipper theories that expand the defendant list. It explains why spoliation is the central battleground and what a court can do about it. Two collisions are followed from the roadside through resolution. Readers finish knowing what to preserve, what to request, and where these cases are won.