Much of the literature on modality focuses, at least implicitly, on the occurrence of single modal auxiliaries. However, cross-linguistically, modal auxiliaries can co-occur with one another, but under interesting restrictions. This thesis examines layered modal constructions and the semantic restrictions under which they combine. For instance, in languages such as Croatian, where double modal constructions are part of the standard, data shows that while some combinations are acceptable, others are not. Therefore, the aim of this thesis is to identify these semantic restrictions and to explain the rationale that motivates them. To answer these questions, a systematic study of four possible combinations (epis- temics embedding epistemics, epistemics embedding non-epistemics, non-epistemics embedding non-epistemics, and non-epistemics embedding epistemics) was carried out. The data shows that the first three groups are, in general, acceptable to native speakers, while the last one is not. Further to that, the data shows that within the non-epistemic + non-epistemic group there seem to be further restrictions. The result was a hierarchical analysis that is based on modal force and flavour. With respect to force, it is shown in Chapter 4 that necessity embeds possibility, crucially, only when two of the same flavour combine. In terms of flavour, the data shows that epistemics can embed non-epistemics, while in the non-epistemic group priority embeds the circumstantial group in which pure possibility embeds ability and disposition, respectively. This analysis carries some important implications for the traditional categorisation of modal flavours which is discussed in Chapter 4. Finally, in Chapter 5 I also discuss the possible rationale behind the hierarchy and the compositional nature of DMCs. It is concluded that the hierarchy should not be taken as a merely descriptive generalisation, but rather as an analysis that is predictable on the basis of the conceptual and logical reality of human language.