responsivity formalism is a reasonably good approximation. Using the measured noise spectra with this responsivity and a conservative quasiparticle lifetime τ$_{qp}$ in aluminum of 250 µ s allows us to calculate the NEP$_{θ}$ [9], where τ$_{res}$ = Q$_{m}$/πf$_{0}$ . This same equation can be used to calculate NEP$_{D}$ by substituting S$_{D}$ for S$_{θ}$ and ∂D/∂N$_{qp}$ for ∂θ/∂N$_{qp}$ . It can also predict the resonator sensitivity in the dissipation direction by using ∂D/∂N$_{qp}$ from the real part of Equation 4 and the normalized voltage noise of an amplifier with noise temperature T$_{n}$ referenced to the center of the resonance loop, Combining Equations 4, 6, and 7 yields the predicted NEP based only on the device properties, shown as the red line in Figure 4. This is a powerful tool for optimizing microstrip MKIDs for specific detector applications. Figure 4 shows the calculated NEP for the resonator measured in Figure 3. As expected, NEP$_{θ}$ is higher than NEP$_{D}$ due to the contribution of TLSs to the phase noise. The measured NEP$_{D}$, with a minimum below 5 × 10 − 17 W Hz − 1 / 2 between 1 Hz and 1 kHz, is a very good NEP competitive with many other low temperature detectors. There is an small unexplained rise in the NEP$_{D}$ at very low frequencies, which may be due to incomplete decomposition of the noise into phase and dissipation. The ease and flexibility of fabrication combined with the good sensitivity makes microstrip MKIDs an extremely interesting device for future large detector arrays. Significantly higher performance in much smaller resonators can be achieved by using thinner films and super- conductors like titanium that have long penetration depths. This material is based upon work supported by the National Aeronautics and Space Administration under Grant NNH06ZDA001N-APRA2 issued through the Science Mission Directorate. The authors would like to thank John Martinis, Sunil Golwala, and Andrew Acknowledgments S pred D = 2 kT$_{n}$ P $^{(}$Q$_{c}$ Q$_{m}$$^{)}$$^{2}$ . (7) 8 NEP 2 $_{θ}$( ω ) = S$_{θ}$ ( ω )$^{(}$ ητ$_{qp}$ Δ$_{0}$ ∂θ ∂N$_{qp}$$^{)}$$^{−}$$^{2}$ (1 + ω $^{2}$τ 2 $_{qp}$)(1 + ω $^{2}$τ 2 $_{res}$) (6)