The Day & Zimmermann is a privately held company in the fields of construction, engineering, staffing and ammunition manufacture, operating out of 150 locations worldwide. Its corporate office is at 1500 Spring Garden Street in Philadelphia, Pennsylvania.

Day & Zimmermann has an annual revenue of $2.5 billion, and is ranked by Forbes as one of the largest privately held companies in the United States.^[1]

The company was founded in 1901 by Charles Day and Kern Dodge, son of James Mapes Dodge, as Dodge & Day,^[3] specializing in engineering, shop equipment and management.^[4] Later, the scope of the organization was enlarged to include a great deal of engineering and construction work in both the industrial and public-service fields.^[5]

In 1907 another former classmate John Zimmerman joined the firm as partner, and they renamed the firm Dodge, Day & Zimmermann. After Kern Dodge withdrew as partner in 1911 the firm became Day & Zimmermann, incorporated in 1916,^[6]

In its early years the company came into prominence with the design of the construction of the Gatun Lock System, one of the Panama Canal locks in 1907. The construction of the Gatun Lock began with the first concrete laid at Gatun, on August 24, 1909, by Day & Zimmermann. In those early days the company was also contracted by the Hershey chocolate company to produce the foil wrapper for Hershey's Kisses.

In 1961, Day & Zimmermann and Yoh Company merged. In the early 1970s it assisted in the construction of Veteran's Stadium in Philadelphia.

In 1976, Spike Yoh bought the company from his father and was the CEO for twenty-two years. He graduated from Duke University and one of Duke's facilities is named Duke's Yoh Football Center. In 1998, his son, Hal Yoh, became the company's leader. In that year Day & Zimmermann was the recipient of the National Family Business of the Year award.

In America the company is one of its leading producers of ammunition, operating several government-owned facilities. In the 2011 financial year, it worked on 351 contracts for the US Army worth 151.8 million dollars.^[7]