  We realize tunable coupling between two superconducting transmission line
resonators. The coupling is mediated by a non-hysteretic rf SQUID acting as a
flux-tunable mutual inductance between the resonators. From the mode distance
observed in spectroscopy experiments, we derive a coupling strength ranging
between -320MHz and 37 MHz. In the case where the coupling strength is about
zero, the microwave power cross transmission between the two resonators can be
reduced by almost four orders of magnitude compared to the case where the
coupling is switched on. In addition, we observe parametric amplification by
applying a suitable additional drive tone.
