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The circumstellar regions of young stellar objects (YSOs) are some of the most interesting environments in astronomy. The leftover material from star formation takes the form of enormous discs of dust and gas extending out to a thousand astronomical units. The outer regions of these protoplanetary discs, being cold and relatively large in scale, have been extensively studied by radio telescopes. Usually shrouded in opaque dust, the regions closer to the stars are best studied in the infrared. Here, the central YSO interacts with the disc, accreting and ejecting material, and planets, asteroids and comets are formed. Understanding how stars interact with their discs can therefore tell us how both evolve, and can help us understand the origin of our own solar system. It also can teach us more about systems very di?erent to our own – such as binary systems. The majority of stars are part of multiple systems, and in recent years we have also found increasing numbers of binary YSOs. The dynamical interaction between the stars and the inner disc can lead to complex features such as spirals, warps and disc truncation, a?ecting their rates of accretion and ejection, as well as a?ecting planetary formation. As the population of binary YSOs grows, we are finding a much larger diversity in the inner regions of their discs than previously thought. Observing these processes has historically been dicult because of the extremely small angular scales on which they occur – on the order of milli-arcseconds – which limits their study with traditional telescopes. In spite of this, the study of young stars and their discs has rapidly become one of the most dynamic areas of astronomy in recent decades. Advances in computing power have allowed for more complex simulations of circumstellar environments. Simultaneously, developments in technology and engineering have substantially increased the set of observable protoplanetary discs, allowing us to rapidly develop and advance ideas of star formation and evolution. The focus of my PhD was on one such technique: near-infrared interferometry. By interfering the light from multiple individual telescopes, it is possible to obtain resolutions far in excess of any single-dish telescope, allowing us to probe inner disc regions with a precision never before possible. This is especially important in the context of close binary YSOs, where dynamic interactions between the stars and the disc can cause large changes to the circumstellar environment which cannot be reliably determined from simulations. In these cases, the feedback from observations of individual systems can be used to calibrate and inform further refinements to the theory. The thesis is laid out as follows. Chapter 1 introduces the general star formation paradigm, including star formation and YSO classification, as well as a discussion on circumstellar discs around single stars. In Chapter 2, I extend this to binary objects, which were the focus of my research. Chapter 3 describes the basic theory behind optical and near-infrared interferometry, its history and the challenges which it poses to anyone wishing to interpret its observables. In this chapter I describe the basic operation of the interferometers which I used over the course of my PhD, the VLTI and the CHARA Array. After this theoretical overview, I present the work which was the focus of my PhD: a deep study into two individual young binary systems. Chapter 4 is dedicated to analysis of observations we took of MWC 166 A, a massive YSO with a combined mass of 17 M located at a distance of 1 kpc. This is a mysterious system with very strong K-band line emission, often associated with disc accretion, but little evidence of a substantial disc itself. In this chapter, I define for the first time a full three-dimensional orbit of the system, which has a period of 368 d, and establish stellar parameters and the evolutionary state of both components, finding that the primary component is likely a main-sequence object while the secondary is just in the process of final contraction. I also constrain the geometry and spectral characteristics of the Br and He i emission lines, and find the most likely origin to be a Be decretion disc in Keplerian rotation. Chapter 5 uses similar techniques to analyse a rather di?erent young binary, HD 104237 A. This T Tauri with a combined mass of 4.3 M lies about ten times closer to us and has a much tighter orbit of 20 d. It also has a much denser inner disc region than MWC 166 A, with both components actively accreting from the circumbinary disc, and this is reflected in the strong but variable Br emission arising from the two components. In this chapter, I derive an orbit for the system, constrain its age and determine fundamental stellar parameters for both components. I also model the circumstellar emission both in the dust continuum, which takes the form of a truncated circumbinary disc, and analyse the rapidly-changing Br emission in the inner region on an epoch-by-epoch basis. This emission shows signs of variable accretion onto both stars, as well as components originating from the circumbinary environment. Both these studies use spectro-astrometric and spectro-interferometric techniques to showcase the usefulness of the high angular resolution a?orded by near-infrared interferometry to gain unique insights into binary-disc interaction, dynamical truncation and binary accretion. This process will only accelerate in future, as new designs of interferometers with even greater precision will unlock an ever-larger tranche of systems, expanding our knowledge of these endlessly-fascinating objects further and further.