There is a group of words which are usually referred to as keisiki-meisi (i.e. formal nouns) in Japanese. The formal noun is defined as a noun which does not have a substantive meaning, and is not used in isolation, but requires a preceding modifier. In this thesis, examinations of various aspects of the Japanese nouns mono and koto, which are widely acknowledged as typical examples of formal noun, will be presented. Mono and koto occur with a variety of terms to produce additional derivatives. For example, when the copula da is attached to mono or koto at the end of a sentence, it becomes a sentence-final modality which strongly reflects the speaker's emotions or feelings. However, owing to the fact that mono and koto are also used on occasion as substantive nouns without preceding modifiers, scholars tend to merely clarify the boundary between the use of mono or koto as a substantive noun, and its use as a formal noun, giving two separate labels to the same noun. In this study, the existence of continuity between these two usages - substantive and formal - is hypothesised. The syntactic and semantic features observed throughout the derivative forms of mono and koto offer a chance to explore and identify the unifying features of the two different usages. It is also demonstrated that, viewed in the light of the framework of grammaticalisation, the category 'formal noun' is only a label that has been put onto a group of nouns which can be grammaticalised or which have already been grammaticalised.