In this paper, we review recent advances in optical parametric oscillators, with particular emphasis on high repetition rate, femtosecond optical parametric oscillators, We present results for several of these systems, and show that they offer a unique combination of short pulse durations, high average powers, high repetition rates, and broad tunability which have not been previously available from a single source, We conclude by looking at some applications in the field of ultrafast semiconductor spectroscopy, some of which are only now possible because of the femtosecond optical parametric oscillator, It is, therefore, apparent that this unique device offers greater flexibility than previously available sources, and will enable many new and novel experiments in many areas of science and technology.