Traditionally, computer interfaces have been confined to conventional displays and focused activities. However, as displays become embedded throughout our environment and daily lives, increasing numbers of them must operate on the periphery of our attention. Peripheral displays, ubiquitous computing devices that present information without demanding attention, are difficult to build, particularly because they must dynamically manage the cognitive load they place on users. We present a toolkit that aids the development of peripheral displays. We determined three key issues for the toolkit, based on a survey of existing peripheral displays and cognitive science literature: abstraction of data, selection of notification levels, and transitions between notification levels. Our contribution is the investigation of these key characteristics, combined with a toolkit that encapsulates them and supports the design of displays that focus on these issues. We describe our toolkit architecture, and present five sample peripheral displays demonstrating our toolkit's capabilities.