  The past three decades have shown dramatic progress in the ability to
manipulate and coherently control the motion of atoms. This exquisite control
offers the prospect of a new generation of inertial sensors with unprecedented
sensitivity and accuracy, which will be important for both fundamental and
applied science. In this article, we review some of our recent results
regarding the application of atom interferometry to inertial measurements using
compact, mobile sensors. This includes some of the first interferometer
measurements with cold $^{39}$K atoms, which is a major step toward achieving a
transportable, dual-species interferometer with rubidium and potassium for
equivalence principle tests. We also discuss future applications of this
technology, such as remote sensing of geophysical effects, gravitational wave
detection, and precise tests of the weak equivalence principle in Space.
