The world’s 20 largest engineering staffing firms account for 39% of the global engineering staffing market valued at $31.0 billion, according to a report by Staffing Industry Analysts.

In the report, the 20 largest firms — each of which has at least $180 million in staffing revenue — are ranked by 2017 global staffing revenue.

Aerotek, a division of Allegis Group, ranked as the largest global supplier of engineering staffing in terms of estimated 2017 global engineering staffing revenue, according to the report. Adecco ranks as second largest, followed by UK-based Fircroft as third.

The top five are:

“This list will help corporate members to understand the structure and size of the global engineering staffing market and how the main players are positioned,” said SIA Senior Research Analyst Matt Norton, the author of the report. “The report will be of use to those looking to identify competitors, suppliers or M&A targets.”

The companies have been ranked in order of revenue size, according to industry custom. SIA corporate members can download the full report online.