YIT Oyj is the largest Finnish and a significant North European construction company. YIT is headquartered in Helsinki and its stock is listed on Nasdaq Helsinki Oy. YIT develops and builds apartments, business premises and entire areas. YIT is also specialised in demanding infrastructure construction and paving. YIT operates in 11 countries: Finland, Russia, Scandinavia, the Baltic States, the Czech Republic, Slovakia and Poland.^[4]

The origins of YIT can be traced back to 1912, when Sweden's General Engineering Company (Swedish: Allmänna Ingeniörsbyrån) established a subsidiary in Finland, initially to operate in the water supply sector.^[5] The Finnish translation of the group's title, Yleinen Insinööritoimisto, is the source of the name YIT. The first project undertaken by the new company was the construction of a water tower in Porvoo.^[6] The firm's field of operation was gradually broadened over the following decades, but it was not until the 1970s that the company moved beyond the civil engineering sector into construction of buildings (including residential property) and industrial works.^[7] In September 1987, Yleinen Insinööritoimisto merged with rival Finnish contractor Perusyhtymä Oy, creating YIT-Yhtymä (YIT Corporation).^[8] Since the merger, YIT's growth into new sectors has been aided by several acquisitions: in 1995, the piping design and maintenance specialist Oy Huber Ab was acquired,^[9][10] the same year that the company's shares were listed on the Helsinki Stock Exchange,^[8] and the Swedish pipeline installation and maintenance firm Calor AB and its Finnish subsidiary were purchased in 2001.^[11] These additions assisted a wider move into the industrial upkeep and maintenance market.^[7] Primatel, the network construction and maintenance arm of the telecommunications firm Sonera, was added in 2002,^[12] and a year later the building systems concerns of ABB in Russia and the Nordic and Baltic regions were acquired, further increasing YIT's presence in the service sector. In 2008 and 2010 YIT has continued expanding its operations into Central Europe.^[13]

In March 2022, YIT quit all activities, such as investments and construction, in Russia because of 2022 Russian invasion of Ukraine.^[14]

YIT's recent acquisitions have seen the company increase markedly in size - both the company's revenue and workforce more than trebled in the ten years to 2007,^[15][16] making YIT the largest construction company in Finland^[17][18] and the largest supplier of building systems services in the Nordic region by sales.^[19] This growth is also reflected in the scope of YIT's business: long-term servicing and maintenance contracts for items such as steam boilers, piping, heating, plumbing and electricity networks and water treatment plants now account for over a quarter of annual revenues.^[20] The Construction Services division, building homes, water and waste treatment plants and infrastructure ranging from railway stations to golf courses remains the group's largest source of revenue as of the end of 2006.^[20]

In June 2017, the company announced its intention to acquire rival Lemminkäinen for €632 million (£553.6 million) in an all-share deal representing a 40% premium over Lemminkainen's current share price.^[21] The new YIT was born when over 100-year-old YIT Corporation and Lemminkäinen Corporation merged on February 1, 2018.

Ehrnrooth family is the biggest owner of the construction company YIT, 12.1% in the end of January 2012.^[25] Henrik Ehrnrooth is the chairman of the YIT Board of Directors since 2009.^[26]

In Bani Walid in Libya, YIT constructed in silence a military factory in 1983. Company changed its name from Perusyhtymä to YIT in 1987.^[27][28][29]

40 residents made complaints of ammonia observed in YIT residential constructions in St. Petersburg in September 2011. Russians add ammonia to concrete in the winter construction.^[30]

YIT demanded €20,000 sanctions for silence from an elderly couple in Espoo in the conflict of construction in 2011.^[37]