The Italian Nationality Law establishes the requirements for Italian citizenship, the application procedure and it also regulates the loss of Italian citizenship.
The main Italian Nationality piece of legislation that regulates citizenship is called Legge 91/1992, and you can find the full text (in Italian) here.
There are a number of other citizenship laws, decrees and court orders that regulate specific aspects of Italian citizenship together with Legge 91/1992, and you can find a list here.
If you are looking for information on the Italian Nationality legislation in English, there’s a great Wikipedia page on the subject. Its draws a very detailed and comprehensive picture of the current laws as well as the history and development of the legislation.