Two very different starting points
If you have been sued, the papers set a response deadline that runs from service, and the first job is getting them to counsel and putting a hold on relevant records. If you own the patent, a complaint generally calls for a reasonable investigation before filing, which usually means comparing the patent claims to the accused product element by element. Infringement claims are heard in federal court or, for imported goods, sometimes before the International Trade Commission. Where a U.S. company can be sued for patent infringement is narrower than in many other kinds of cases. The identity of the parties matters too, since a competitor fighting over market share and a company whose business is licensing patents tend to want different things.
A parallel track at the USPTO
A defendant may ask the Patent Trial and Appeal Board to review the validity of the asserted claims in an inter partes review, which runs on its own record and its own deadlines. Whether a petition is taken up is a matter of the office's discretion, and its practice on that question has shifted, so it should not be treated as a sure option. Courts sometimes pause a case while such a review runs and sometimes decline to. A patent owner should expect its claims and prosecution history to be examined closely on both tracks. What a party says about claim meaning in one forum can be used against it in the other.
Records and costs to address now
Preserve design files, source code, engineering notes, sales data, and email about the product and the patent, and hold off on internal commentary about whether the product infringes. Look at supplier contracts for indemnity terms and give prompt notice to any insurer, keeping in mind that general liability policies usually exclude patent claims. Patent litigation is expensive, and we talk about budget, phasing, and possible early resolution from the start. In a first meeting we map what is accused, which claims are at issue, the forum, and the decisions that cannot wait.
