Technological progress in integrated, low-power, CMOS communication devices and sensors makes
a rich design space of networked sensors viable. They can be deeply embedded in the physical world
or spread throughout our environment. The missing elements are an overall system architecture and a
methodology for systematic advance. To this end, we identify key requirements, develop a small device
that is representative of the class, design a tiny event-driven operating system, and show that it provides
support for ecient modularity and concurrency-intensive operation. Our operating system ts in 178
bytes of memory, propagates events in the time it takes to copy 1.25 bytes of memory, context switches
in the time it takes to copy 6 bytes of memory and supports two level scheduling. The analysis lays a
groundwork for future architectural advances.