Book excerpt: Due to its impressive sensitivity, long baseline atom interferometry is an exciting tool for tests of fundamental physics. We are currently constructing a 10-meter scale apparatus to test the Weak Equivalence Principle (WEP) using co-located Rb85 and Rb87 atom interferometers. This apparatus aims to improve the current limit on WEP violation 100-fold, which illustrates the power of this technique. This scientific goal sets stringent requirements on the kinematic preparation of the atomic test masses, the interferometer laser wavefront and stability, as well as the electromagnetic and gravitational field homogeneity of the interferometer region. The efforts to control these sources of systematic error are discussed. Additionally, applications of long baseline atom interferometry to space-based sensors for geodesy and gravitational wave detection are presented.