Materials systems with many strongly interacting degrees of freedom can host some of the most exotic physical ground states known. The subtle interplay of Coulomb interactions, electron-lattice coupling and spin/orbital ordering gives rise to phenomena as diverse as high-temperature superconductivity and topological insulating states as well as ferroelectricity and ferromagnetism. Many of these emergent ground states are found in perovskite oxide crystals where the close lattice matching across the series further permits abrupt heterointerfaces to be formed. Here I will show how advanced thin film deposition, in conjunction with analytical electron microscopy, can be used to engineer novel multifunctional complex oxide materials. In particular, I will discuss the design of the first material shown to be a strong magnetoelectric multiferroic at room-temperature, a promising candidate for low-power electronics in which electric fields control magnetic states.