[[File:Unison strike rally Oxford 20060328.jpg|thumb|right|250px|A rally of the trade union [[UNISON]] in [[Oxford]] during a strike on 28 March 2006.]]
[[File:Day113kstreetb.JPG|thumb|right|250px|[[2005 New York City transit strike]]]]
Most strikes are undertaken by [[trade union|labor unions]] during [[collective bargaining]] as a last resort. The object of collective bargaining is for the employer and the union to come to an agreement over wages, benefits, and working conditions. A [[collective bargaining agreement]] may include a clause (a contractual "no-strike clause") which prohibits the union from striking during the term of the agreement.<ref name=FeldackerHayes>{{cite book|title=Labor Guide to Labor Law|author=Bruce S. Feldacker & Michael J. Hayes|date=2014|publisher=Cornell University Press|date=2014|pages=231, 244-46}}</ref> Under U.S. labor law, a strike in violation of a no-strike clause is not a [[protected concerted activity]].<ref name=FeldackerHayes/> The scope of a no-strike clause varies; generally, the U.S. courts and [[National Labor Relations Board]] have determined that a collective bargaining agreement's no-strike clause has the same scope as the agreement's arbitration clauses, such that "the union cannot strike over an arbitrable issue."<ref name=FeldackerHayes/> The U.S. Supreme Court held in ''Jacksonville Bulk Terminals Inc. v. International Longshoremen's Association'' (1982), a case involving the [[International Longshoremen's Association]] refusing to work with goods for export to the [[Soviet Union]] in protest against its [[Soviet–Afghan War|invasion of Afghanistan]], that a no-strike clause does not bar unions from refusing to work as a political protest (since that is not an "arbitrable" issue), although such activity may lead to damages for a [[secondary boycott]].<ref name=FeldackerHayes/> Whether a no-strike clause applies to [[sympathy strikes]] depends on the context.<ref name=FeldackerHayes/> Some in the labor movement consider no-strike clauses to be an unnecessary detriment to unions in the collective bargaining process.<ref>{{cite web|url=http://labornotes.org/blogs/2011/12/no-strike-clauses-hold-back-unions|title=No-Strike Clauses Hold Back Unions – Labor Notes|website=labornotes.org|date=2011-12-13}}</ref>
Occasionally, workers decide to strike without the sanction of a labor union, either because the union refuses to endorse such a tactic, or because the workers concerned are non-unionized. Such strikes are often described as ''unofficial''. Strikes without formal union authorization are also known as [[Wildcat strike action|wildcat strikes]].
