Quantitative phase imaging (QPI) is a category of imaging techniques that can retrieve the phase information of the sample quantitatively. QPI features label-free contrast and non-contact detection. It has thus gained rapidly growing attention in biomedical imaging. Capable of resolving biological specimens at tissue or cell level, QPI has become a powerful tool to reveal the structural, mechanical, physiological and spectroscopic properties. Over the past two decades, QPI has seen a broad spectrum of evolving implementations. However, only a few have seen successful commercialization. The challenges are manifold. A major problem for many QPI techniques is the necessity of a custom-made system which is hard to interface with existing commercial microscopes. For this type of QPI techniques, the cost is high and the integration of different imaging modes requires nontrivial hardware modifications. Another limiting factor is insufficient sensitivity. In QPI, sensitivity characterizes the system repeatability and determines the quantification resolution of the system. With more emerging applications in cell imaging, the requirement for sensitivity also becomes more stringent. In this work, a category of highly sensitive full-field QPI techniques based on wavelength shifting interferometry (WSI) is proposed. On one hand, the full-field implementations, compared to point-scanning, spectral domain QPI techniques, require no mechanical scanning to form a phase image. On the other, WSI has the advantage of preserving the integrity of the interferometer and compatibility with multi-modal imaging requirement. Therefore, the techniques proposed here have the potential to be readily integrated into the ubiquitous lab microscopes and equip them with quantitative imaging functionality. In WSI, the shifts in wavelength can be applied in fine steps, termed swept source digital holographic phase microscopy (SS-DHPM), or a multi-wavelength-band manner, termed low coherence wavelength shifting interferometry (LC-WSI). SS-DHPM brings in an additional capability to perform spectroscopy, whilst the LC-WSI achieves a faster imaging rate which has been demonstrated with live sperm cell imaging. In an attempt to integrate WSI with the existing commercial microscope, we also discuss the possibility of demodulation for low-cost sources and common path implementation. Besides experimentally demonstrating the high sensitivity (limited by only shot noise) with the proposed techniques, a novel sensitivity evaluation framework is also introduced for the first time in QPI. This framework examines the Cramér-Rao bound (CRB), algorithmic sensitivity and experimental sensitivity, and facilitates the diagnosis of algorithm efficiency and system efficiency. The framework can be applied not only to the WSI techniques we proposed, but also to a broad range of QPI techniques. Several popular phase shifting interferometry techniques as well as off-axis interferometry is studied. The comparisons between them are shown to provide insights into algorithm optimization and energy efficiency of sensitivity. / Doctor of Philosophy / The most common imaging systems nowadays capture the image of an object with the irradiance perceived by the camera. Based on the intensity contrast, morphological features, such as edges, humps, and grooves, can be inferred to qualitatively characterize the object. Nevertheless, in scientific measurements and research applications, a quantitative characterization of the object is desired. Quantitative phase imaging (QPI) is such a category of imaging techniques that can retrieve the phase information of the sample by properly design the irradiance capturing scheme and post-process the data, converting them to quantitative metrics such as surface height, material density and so on. The imaging process of QPI will neither harm the sample nor leave exogenous residuals. As a result, it has thus gained rapidly growing attention in biomedical imaging. Over the past two decades, QPI has seen a broad spectrum of evolving implementations, but only a few have seen successful commercialization. The challenges are manifold whilst one stands out - that they have expensive optical setups that are often incompatible with existing commercial microscope platforms. The setups are also very complicated such that without professionals having solid optics background, it is difficult to operate the system to perform imaging applications. Another limiting factor is the insufficient understanding of sensitivity. In QPI, sensitivity characterizes the system repeatability and determines its quantification resolution. With more emerging applications in cell imaging, the requirement for sensitivity also becomes more stringent. In this work, a category of highly sensitive full-field QPI techniques based on wavelength shifting interferometry (WSI) is proposed. WSI images the full-field of the sample simultaneously, unlike some other techniques requiring scanning one probe point across the sample. It also has the advantage of preserving the integrity of the interferometer, which is the key structure to enable highly sensitive measurement for QPI methods. Therefore, the techniques proposed here have the potential to be readily integrated into the ubiquitous lab microscopes and equip them with quantitative imaging functionality. Differed by implementations, two WSI techniques have been proposed, termed swept source digital holographic phase microscopy (SS-DHPM), and low coherence wavelength shifting interferometry (LC-WSI), respectively. SS-DHPM brings in an additional capability to perform spectroscopy, whilst the LC-WSI achieves a faster imaging rate which has been demonstrated with live sperm cell imaging. In an attempt to integrate WSI with the existing commercial microscope, we also discuss the possibility of demodulation for low-cost sources and common path implementation. Besides experimentally demonstrating the high sensitivity with the proposed techniques, a novel sensitivity evaluation framework is also introduced for the first time in QPI. This framework not only examines the realistic sensitivity obtained in experiments, but also compares it to the theoretical values. The framework can be widely applied to a broad range of QPI techniques, providing insights into algorithm optimization and energy efficiency of sensitivity.