Future quantum networks will enable fascinating applications such as secure communication, quantum-enhanced distributed sensing, distributed quantum computing, and accurate global timekeeping. Center to these applications is quantum entanglement distribution, whose effective distance is limited by photon loss. Quantum repeaters were proposed to extend the effective distance, but their demonstration has not been reported yet. In this thesis, we study quantum repeaters with both single-emitter based and ensemble-based quantum memories. In particular, we study the feasibility of meaningful proof-of-principle demonstrations of several quantum repeater protocols (with two-links) with photon (single-photon and photon-pair) sources and atomic-ensemble based quantum memories. We take into account non-unit memory efficiencies that decay exponentially with time, which is more realistic and accurate compared to previous treatments. We discuss implementations based on quantum dots, parametric down-conversion, rare-earth-ion doped crystals, and Rydberg atoms. Our results provide guidance for the near-term implementation of long-distance quantum repeater demonstrations, suggesting that such demonstrations are within reach of current technology.