Generally, the defendant, once having been served with a Summons and Complaint, is required to file with the Court and serve upon the plaintiff or plaintiff's counsel a response to the allegations in the Complaint. This response is known as an Answer.
There is a very limited time within which to file, so you may want to review this with an attorney to ensure you’re keeping up with the requirements. The attorney would also be able to help determine what defenses should be used and whether or not Counterclaims should be asserted.
If it is a Small Claims action, then you typically receive a copy of the Summons and Complaint form that includes an initial appearance date. In Small Claims cases you may, but are not required to, file and serve a formal written Answer - it may be pled orally. Essentially, at the initial appearance, you simply indicate that the allegations are contested, and a return date is scheduled.
For a brief discussion of the basic steps of a lawsuit in Wisconsin, see the informational video series found in my profile, or visit this blog for more informational videos: http://law-wi.blogspot.com/
I am going to add some information to Kevin Davidson's Response in case the summons and complaint is not small claims. To tell if a case is small claims, look at the case number. If it looks like 09 SC 0000123, the SC tells you it is small claims. If it looks like 09 CV 000123, it is a civil suit.
1. The summons will tell you how long you have to respond. Count the days from the date you were served. The answer must be given to or mailed to the plaintiff's attorney (or plaintiff if there is no attorney) within the period to respond.
2. You must respond to each allegation of facts. Often allegations are numbered. Sometimes not. The response may be to admit, deny, admit in part, deny in part, or that you lack sufficient information to respond.
3. If you have any affirmative defenses, you must list these as well.
4. If your corporation or limited liability is being sued, you cannot respond yourself, but you must have an attorney represent you.
5. Help your attorney by drafting a response and taking it with you when you see your attorney.
If you do not have an attorney, you can choose one on Avvo, or you can contact the Wisconsin Bar Association Lawyer Referral and Information Service.