Atomic vapors of alkali metals are widely used to slow and stop light in tabletop experiments. In order to take advantage of the underlying quantum interference effects in future commercial devices, highly reactive alkali atoms must be incorporated into small volumes with integrated optical access. With integration in mind, we describe the development of a hollow-core waveguide technology based on the combination of vapor-filled hollow waveguides and conventional solid-core waveguides on a silicon chip. We discuss the underlying principles of the waveguide design, the development of different approaches to building on-chip vapor cells, the demonstration of linear and nonlinear rubidium spectroscopy on a chip, and the prospects for quantum interference effects such as slow light and giant Kerr nonlinearities using this approach. Ultrasmall active vapor volumes on the order of 100 picoliters with simultaneously high optical density in excess of two illustrate the potential of planar hollow-core waveguides for linear and nonlinear optical spectroscopy of atoms confined on a chip.