Briefly:

In 2013 there were 14.5 million members in the U.S., compared with 17.7 million in 1983. In 2013, the percentage of workers belonging to a union in the United States (or total labor union “density”) was 11.3%, compared to 20.1% in 1983.[1] From a global perspective, the density in 2010 was 11.4% in the U.S., 18.4% in Germany, 27.5% in Canada, and 70% in Finland.[2] Union membership in the private sector has fallen under 7%[3] — levels not seen since 1932.

In the 21st century the most prominent unions are among public sector employees such as city employees, government workers, teachers and police. Members of unions are disproportionately older, male, and residents of the Northeast, the Midwest, and California.[4] Union workers average 10-30% higher pay than non-union in the United States after controlling for individual, job, and labor market characteristics.[5] (Wikipedia)

Here’s a graph: