Deep submicron technologies are beginning to scale poorly with respect to both power and performance. It is well known that adding timing assumptions to asynchronous circuits can help to simplify circuits and improve performance. Thus, applying timing assumptions can help to extend the effectiveness of technology scaling. However, employing timing assumptions in deep submicron technologies is risky because of the large process variations that are present. This thesis explores the use of low risk timing assumptions to improve asynchronous circuits. We begin with a well-established and robust asynchronous logic style, quasidelay insensitive (QDI) circuits. We expose a timing assumption that exists in the feedback of QDI circuits and extend it for general use. We refer to the resulting logic family as relaxed quasi delay-insensitive circuits (RQDI). RQDI circuits maintain much of the robustness of QDI circuits while providing improved power and performance. Evaluations show that replacing QDI circuits with RQDI equivalents can reduce area and energy by 20% and 36%, respectively. RQDI also allows for new types of circuits which are difficult to design using strictly QDI logic. We present RQDI circuits for voltage scaling and two phase signaling. The voltage scaling circuits are novel because they allow for independent voltage scaling of the forward path (data rails) and the return path (acknowledges). The two phase circuits are presented in the context of static switching networks, such as those found in the routing networks in a field-programmable gate array (FPGA). Evaluations show that our two phase circuits can reduce energy con- sumption in these structures by more than 50% with an area overhead of less than 10%. To further evaluate RQDI circuits, we design an asynchronous FPGA using RQDI two-phase circuits and RQDI voltage scaling circuits. For eight of the MCNC LGSynth93 benchmarks, RQDI two-phase circuits provide up to a 70 % performance improvement and up to a 40 % power reduction. The RQDI voltage scaling circuits provide an additional 30 % power reduction across these benchmarks.