In this thesis, the focus was on understanding recent developments in liquid chromatography (LC) and how these can be applied to improve two-dimensional separations (Chapter 2). The main advances that were of interest for the design of a 3D separation device were those in column technology, instrument capabilities and miniaturized separation devices. We proceeded with an investigation into the miniaturization of comprehensive two-dimensional LC (LC×LC) for the analysis of peptides by direct coupling with a high-resolution mass spectrometer (HR-MS) (Chapter 4). In choosing the best combination of retention mechanisms for the LC×LC system, we have investigated computational programs for the prediction of retention and the optimization of peptide separations in hydrophilic-interaction liquid chromatography (HILIC) and reversed-phase liquid chromatography (RPLC) (Chapter 3). Finally, we investigated the introduction of particles in a 3D-printed device as a model for a future 3D-spatial-separation device (Chapter 5). To reach the ultimate goal of a 3D-spatial-separation device with the capability of achieving a peak capacity of one million, further research is still required. The future outlook following the research in this thesis is described in Chapter 6