Superconducting quantum circuits are a promising platform for quantum computation. The building block for most quantum processors is a qubit (quantum bit) which can store information in a superposition of two states. Superconducting qubits are lithographically defined from metals, often niobium or aluminum. However, these devices have limited use because the information they store decays before most useful computations can take place. In this thesis we explore the cause of these losses. Specifically, we employ tantalum as the capacitor pad of a two-dimensional transmon qubit and find lifetimes and coherence times with dynamical decoupling over 300 µs. We then switch to a resonator geometry to probe tantalum materials properties. We develop a power and temperature dependent measurement to quantify sources of decay. We find our resonators are primarily limited by two-level system loss at materials interfaces. Finally we employ this resonator char­ acterization method to determine the effects of processing treatments and new packages on resonator decay, showing a buffered-oxide etch before measurement reduces two-level system loss.