In this thesis we describe investigations of a novel class of semiconductor-glass waveguide hybrid lasers. We show that such lasers provide an unprecedented spectral purity as well as tunability among all chip-based diode lasers. The concept of the investigated hybrid lasers is based on spectral control of light generated with a laser diode using highly frequency selective, tunable optical feedback provided by an integrated photonic circuit. In this work, we have employed InP laser diodes that generate light at around 1.5 μm wavelength, as an important as well as representative example. The integrated waveguide feedback circuits are based on microring resonators (MRRs). The waveguide platform selected for the feedback circuits utilizes Si3N4 as the waveguide core material and SiO2 as cladding material (TriPleXTM), because this provides exceptionally low propagation loss in combination with high index contrast, to enable large optical path lengths (for linewidth narrowing) and sophisticated circuit geometries in a compact, chip-sized format. The thesis is organized as follows: after presenting some theoretical background in Chapter 2, we present a first version of a hybrid laser that possesses reduced roundtrip losses as achieved with an improved optical coupling between the diode and silicon nitride waveguide mode fields. To explore the potential of using narrowband amplification, frequency and phase synchronization, such as for application in the emerging field of microwave photonics. In Chapter 4 we report the first injection locking experiments with such hybrid lasers; in Chapter 5 we report the first integration of a hybrid InP-Si3N4 laser; in Chapter 6 we present a theoretical analysis of such hybrid lasers that for the first time reveals index-induces spatial broadening effects with a spatially resolved modeling of the gain section. Finally, in Chapter 7 we describe how the expertise gained with modeling, laser-design and experimental investigation is used to realize a semiconductor-glass hybrid laser with a record-low Schawlow-Townes linewidth of 290 Hertz. In brief, this thesis describes a most powerful approach towards spectral control of diode lasers for unprecedented high spectral purity, and with large relevance for high-impact applications.