Winning liability and proving damages are separate projects, and lawyers lose the second one far more often than the first. This article explains how damages are actually proved: what categories exist, what evidence each requires, and why the reasonable certainty requirement defeats more claims than any defense. It works through contract damages and the expectation measure, tort damages and the distinction between economic and non-economic loss, the mitigation duty that shifts the burden to the defendant, and the causation link that must connect the wrong to each dollar claimed. It explains punitive damages and the constitutional limits on them, statutory damages and why they exist, and the interest and fee rules that decide what a judgment is finally worth. It covers the practical mechanics — experts, models, documents, and the offer of judgment that can shift costs — and closes with the collection problem that turns a paper judgment into money or into nothing.