  We report on the design and performance of Microwave Kinetic Inductance
Detectors (MKIDs) sensitive to single photons in the optical to near-infrared
range using hafnium as the sensor material. Our test device had a
superconducting transition temperature of 395 mK and a room temperature normal
state resistivity of 97 $\mu \Omega$ cm with an RRR = 1.6. Resonators on the
device displayed internal quality factors of around 200,000. Similar to the
analysis of MKIDs made from other highly resistive superconductors, we find
that modeling the temperature response of the detector requires an extra
broadening parameter in the superconducting density of states. Finally, we show
that this material and design is compatible with a full-array fabrication
process which resulted in pixels with decay times of about 40 $\mu$s and
resolving powers of ~9 at 800 nm.
