A union is a worker association that bargains with employers over wages, benefits, and working conditions. Whereas only 13 percent of US. workers now belong to unions, unions played a much larger role .in the US. labor market in the past. In the 1940s and 1950s, when unions were at their peak, about a third of the US. labor force was unionized. Moreover, unions continue to play large role in many European countries. In Sweden and Denmark. for instance, more than three-fourths of workers belong to unions .