  We have prepared the newly discovered Fe-based superconducting material $\rm
LaO_{1-x}F_xFeAs$ ($0\leq x\leq 0.2$) in polycrystalline form and have
investigated the samples by means of structural, thermodynamic and transport
measurements. Our investigations reveal a non superconducting phase at $0\leq
x\lesssim0.04$ which for $x=0$ is characterized by a structural transition
towards an orthorhombic distortion at $T_s\approx 160$ K and antiferromagnetic
spin order at $T_N\approx138$ K. Both transitions lead to strong anomalies in
various transport properties as well as in magnetization and in specific heat.
Remarkably, the transition temperatures are only weakly doping dependent up
$x\lesssim 0.04$. However, the transitions are abruptly suppressed at
$x\geq0.05$ in favour of a superconducting phase with a critical temperature
$T_c\gtrsim 20$ K. Upon further increasing the F-doping $T_c$ increases up to a
maximum of $T_c=26.8$ K at $x=0.1$ which is followed by a decrease down to
$T_c\approx10$ K at $x\geq0.15$.
