The Library classifies its material according to the U.S.A Congress Library Classification System. For medical subjects uses the NLM classification.

According to this system, human knowledge is classified in 26 subject headings (classes), based on the letters of the Latin alphabet. Each one of them is subdivided hierarchically to smaller categories.

A	General Works
B-BD	Philosophy
BF	Psychology
BH-BJ	Aesthetics, Ethics
BL BX	Religion
C	Auxiliary Sciences of History
D	History: General and Old World
DA	Great Britain
DAW	Central Europe
DC	France
DD	Germany
DE	Mediterranean, Greek-Roman World
DF	Greece
DG	Italy
DH	Benelux [,Be(lgium) - Ne(therlands) - (Lux(enburg)]
DJ	Holland
DJK	Eastern Europe
DK	Soviet Union, Poland
DL	North Europe (Denmark, Iceland, Norway, Sweden, Finland)
DP	Spain, Portugal
DQ	Switzerland
DR	Balkans(Bulgaria, Albania, Yugoslavia, Romania, Turkey)
DS	Asia
DT	Africa
DU	Australia
E	America
F	U.S.A
G	Geography, Anthropology, Anthropography, Folklore, Sports, Recreation
H	Social Sciences
HA-HJ	Economics
HM-HV	Sociology
HX	Socialism
J	Political Science
K	Law
L	Education
M	Music
N	Fine Arts
P	General Philology
PA	Linguistics
PA 1 - 2.915	Greek and Latin language
PA 3.051 - 4.500	Ancient Greek Literature
PA 5.000 - 5.600	Byzantine Philology and Literature
PA 6.000 - 8.595	Latin Philology and Literature
PAA	Modern Greek Literature
PB-PM	Modern European & Other Languages
PN	Literature, Theatre (General)
PQ	French, Italian, Spanish, Portuguese Literature
PR	English Literature
PS	American Literature
PT	German, Dutch, Flemish, Scandinavian Literature
Q	Science
QA-QB	Mathematics, Astronomy
QC	Physics
QD	Chemistry
QH-QM	Natural Sciences (Botany, Zoology, Anatomy)
QP	Neuropsychology, Neurophysiology
R	Medicine
RC	Psychiatry
RJ	Pediatrics
S	Agriculture
T	Technology
U-V	Military and Naval Science
Z	Paleography, Library Science, Library Catalogs, National Bibliographies, Manuscripts' Lists, Bibliography