Hence, Mommsen conceived the Corpus Inscriptionum Latinarum (CIL), an ambitious project (still in progress) to transcribe and review Latin inscriptions and make them available to academics. The CIL has ever since also contained images, something quite revolutionary at the time.
When Mommsen began to plan the CIL, he realised that he also had to consider the codex, given that some inscriptions survived only as copies.
By virtue of being a librarian in the Vatican Apostolic Library, and to his constant connections with other people involved, Giovanni Battista de Rossi was an important authority who took part in this project. 

  In order to understand Mommsen’s revolution in history and its reasons, we must first take a brief look at the history of epigraphic manuscripts and at their cultural importance during the centuries before Mommsen’s intervention.

At the beginning of the Middle Ages, apart from Late Imperial texts describing Rome, regional catalogues and breviaria; Christian pilgrim texts appeared – whereby epigraphs were copied.
Currently, the oldest known codex, dating from the 8th century, contains a collection of Roman inscriptions.
For a certain period, there had been a flourishing circulation of different collections of inscriptions; even in the Corpus Laureshamensis there is a series of Roman epigraphs with the same purpose of providing descriptions.
During the following period, between the 11th and 12th century, there was a lack of interest in producing similar works. This was probably due to the difficulty in understanding the abbreviations and formulae, and the state of preservation of the inscriptions. Nonetheless, in manuscripts of this period it is common to find transcriptions of epigraphic texts in the margins.
From the 14th century onwards, there had been a growing interest in the Ancient World and a corresponding renewal of interest in epigraphs. This newfound love for the Roman world was combined with a revolution in writing inspired by Roman forms. Inscriptions were copied either on commission, or out of ‘disinterested love’ on behalf of humanists. A case in point is the Silloge Signoriliana, which later became the Descriptio urbis Romae after being assigned by Pope Martin V.
During the 15th century similar collections of epigraphic texts continued to be produced with the intention of spreading knowledge about Roman antiquities. The most important of this century is the Codex Balbani.
A different method of approach is used in the epigraphic collection of Cyriacus of Ancona, son of merchants and merchant himself. He transcribed a huge number of inscriptions that he found not only in Italy but also, during his travels, in Dalmatia, Epirus, Greece, Egypt and Asia Minor. He was the first to transcribe such a large number of inscriptions, creating a new literary genre: ancient epigraphic manuscripts.
Unfortunately, the Commentarii were burnt to ashes, but luckily the texts were already in circulation among scholars who made great use of them.
After Cyriacus’ work, epigraphic collections focused not only on Rome but on the rest of Italy, with personal inspection of the sites and drawings.
By following the rules described above, even Lorenzo the Magnificent, who had become an icon of the Renaissance, was given an epigraphic collection of Latin and Greek inscriptions, and achieved great success. Other important collections were created by renowned Renaissance artists but were never printed. These collections were created not only for the pleasure of antiquity but also because they were highly considered historical and literary sources. In fact, they were used to fix a word that had eroded over the years, and to establish the correct context of the use of the word itself.
The invention of printing led to the printed publication of a few collections, the first of which focused on the city of Rome.

    Mommsen himself explains the importance of these texts:

With limited exceptions, inscriptions do not belong to literature but to life; dealing with them provides us with knowledge of antiquity, with a profit similar to that which travelling in a country brings to the bookish knowledge of a country. We come to perceive what the writer, especially if they are a native, seldom portrays, the common and daily course of life. In addition, beneath the dull and futile things, the features emerge from time and space; the language reaches our ears not merely as it should, but as it actually is in its numerous individual changes. We learn a great deal that may be of no great value but is gladly learnt precisely because it was not intended to reach posterity. In short, the treasure of inscriptions, if properly used, is much more than a summary of information. It is – along with the picture that ancient literature has handed down to us of that marvellous age – its true reflection, which without pretence of demand and art shows the flatness, triviality and the simplicity and greatness of antiquity. And with its immediacy, illuminates the stylised or mannered tradition in the right light, not infrequently for the first time.

Mommsen, after his lecture on the artform of Gracchi, was asked to write Römische Geschichte. A work that due to its style and historical breakthrough was widely criticised by experts of the time. If we think of history, of how it is conceived and studied now, we cannot but think of this German academic.

     Mommsen used a simple, plain language as opposed to the courtly, academic one. His work is full of fierce and desecrating judgements: Cicero was treated as a journalist of the worst kind, Pompey as (only) a brilliant cavalry officer.
His nature as an animal politicum – which caused him to lose his professorship in civil law and embitter his relationship with Otto von Bismarck –  influenced Mommsen’s historical vision. His radical liberalism led him to coincide the end of Roman history with the Battle of Thapsus with the end of republican freedom.
Indeed, in his Römisches Staatsrecht, Mommsen also traced out an idea of a modern, liberal Roman state, the state that was so yearned for during the second half of the nineteenth century and the first half of the twentieth century, leading Mommsen to look for it in Rome.

     He was undoubtedly a ‘modernising’ writer, but in his work there is an inner core that lives a life of its own, oblivious of the static mould in which it is set, it is the polyphony that resounds restlessly and irreverently.
In his notes, Mommsen included a series of elements that contradicted his thesis; he created interpretative models but analysed and commented on all the material: in his passages there are hints that open up new paths that turn his work upside down but still lead back to his vision.

His work thus became indispensable for any academic wishing to delve into the fields already covered by him.

     He brought about a revolution in German historiography. If previously the focus was on political, diplomatic and institutional history, Mommsen began to consider the masses, social processes and the economy. He changed the interpretation of Roman history from a solipsistic view to an inscription of Rome in the Italian and, later, imperial context.
In addition to the CIL, his texts, his commentaries on numismatics and his Corpus Agrimensorum Romanorum are fundamental to understanding how Mommsen interpreted the historical importance of every aspect of daily life.

Theodor Mommsen was, above all, an academic and during his life he published more than 1500 works. Yet he was never satisfied; shortly after receiving the Nobel Prize, he died in 1903, leaving a codicil in his will.

I ask my parents to prevent, insofar as possible, the publication of extensive biographies after my death, and especially to avoid providing documents for this purpose. In spite of my victories, I have not achieved what I should have in life. External circumstances have transferred me to the ranks of historians and philologists, although my training and my intellect were certainly not sufficient for these two disciplines. The painful feeling of the inadequacy of my work, of seeming more than being, has never left me throughout my life, and in a biography it should neither be concealed nor highlighted. Let me add another consideration. I have never had and never coveted any political position or influence; but in my heart, and I believe so with what is best in me, has always been one of an animal politicum, wishing to be a citizen. This is not possible in a nation in which even the best individual does not transcend service in the ranks and political fetishism. This internal estrangement from the people to whom I belonged to induced me in every way to evade presenting myself with my own personality, as far as I could to the German people, whom I do not esteem. I wish that even after my death they needn’t be bothered with my individuality. My books can be read as long as they last; what I have been or should have been does not concern these people.