Broadly speaking, there is rather less documentary material on the subject of this study available from Catholic sources than there is from the Italian State. The Vatican authorities do not permit access to papers in the Secret Archives which date from after 1903 (the death of Leo XIII), thus excluding all material from the period under consideration. A part of this regrettable gap has been filled by the work of two historians who, by virtue of their ecclesiastical status, have been exempted from the general prohibition: Mons. Maccarone's publication of the diary of Prince Francesco Pacelli provides a unique, blow-by-blow account of the negotiations which produced the Lateran Pacts of 1929 by the Vatican's chief negotiator, and Padre Angelo Martini whose work Studi sulla Questione Romana e la Conciliazione reproduces documents from the Vatican Archives, unavailable elsewhere, relating to the negotiations which brought the crisis of 1931 to an end. In addition the Documenti Diplomatici Italiani proved to be absolutely indispensable for the whole period and for 1931 in particular.

As well as having to cope with the handicap of not being allowed access to the Vatican Archives, the historian of this period also faces the problem of having only a very limited amount of biographical material, most of it of dubious value, on the leading Vatican personalities involved in the events described – Pius XI, Cardinals Gasparri and Pacelli, Monsignori Borgoncini- Duca and Pizzardo, and Padre Tacchi-Venturi.