This PhD thesis examines a phenomenon known as Monosyllabic Circumflexion (MC, hereafter) from a historical linguistics / phonological point of view. MC denotes a Lithuanian or Balto-Slavic phenomenon according to which long vowels and diphthongs in monosyllabic words exhibit a circumflex tone instead of the expected acute tone.  It is observed in the following four categories: I. 3rd person future forms of monosyllabic stems (e.g., šõks ― šókti `to jump;' vy͂s ― výti `to drive') II. reflexes of PIE root nouns (e.g., Latv. gùovs `cow;' Lith. šuõ `dog') III. prepositions/adverbs (e.g., nuõ `from' ~  nùotaka `bride;' vė͂l `again' ~ Latv. vêl `still, yet,' tė͂ (permissive particle) < *teh1) IV. pronominal forms (e.g., tuõ ~ gerúoju `the good (m.~sg.~instr.),' tie͂ ~ tíeji `id. (pl.nom)'). The unexpected circumflex tone in these categories is problematic and important for the solution of a Balto-Slavic accentological question on the etymological background of acute and non-acute tones. The aim of this thesis is to partially contribute to the solution of this problem by establishing the existence of MC and its relative chronology. The first category, the 3rd person future forms, provides a substantial number of examples and counterexamples. The examination of them has revealed the fact that the counterexamples constitute a morpho-semantic group of verbs whose future stems underwent considerable morphological changes in the prehistory, hence not exhibiting MC. This shows that the regular tonal reflex of the 3rd person future forms of monosyllabic acute stem must be circumflex, allowing for the establishment of MC as a regular phonological process, although this category does not provide much information on the relative chronology of MC. The second category, the reflexes of Proto-Indo-European root nouns, gives an important clue as to where MC is located in the relative chronology of Balto-Slavic sound changes. Next, there is a discussion of whether the results of the examinations of the first two categories can be maintained for the data of the third and fourth categories, which show an irregular distribution of the acute and circumflex tones in monosyllabic forms. It is shown that various morphological factors, such as homonymic clashes within the paradigms for pronouns, can explain why some monosyllabic forms have acute tone. Also, the linguistic feature of West Aukštaitian dialects of Lithuanian that tend to preserve the results of MC is revealed. These dialects are known to have played an important role in the formation of standard Lithuanian. In this way, the monosyllabic forms with unexpected circumflex tone in Lithuanian are explained as a combination of MC in the Proto-Balto-Slavic time and the dialectal tendency of West Aukštaitian dialects of Lithuanian.