Flux qubits are among the first qubits that were ever demonstrated. They have some advantages when compared to capacitively shunted charge qubits, which are now commonly used for building prototypes of quantum processors. Specifically, flux qubits are intrinsically nonlinear systems and they remain so even with low charging energies, which is important for the suppression of large charge noise in solids. In spite of the clear advantages of flux qubits, their applications in multi-qubit devices—prototypes of quantum computers and simulators—are still limited. Flux qubits are also a very powerful tool for fundamental research. In this paper, we discuss the basic properties of flux qubits using the radio frequency superconducting quantum interference device geometry—the most fundamental realization of flux qubits. We also compare and analyze experimental realizations of flux qubits and propose further directions for research.