Ubiquitous computing has a number of compelling applications ranging from biomedical sensing to environmental monitoring. These computing systems require low cost sensor nodes with volumes ≪1mm³ and lifetimes on the order of months or years. We advocate the use of aggressively scaled supply voltages in such applications to maximize energy efficiency. This paper reviews our recent progress in mapping out the low energy design space. We explore the design and test of three low voltage systems targeting ubiquitous computing. We conclude with a survey of open research directions in the ultra-low energy design space.