The spatial development of planar incompressible countercurrent shear layers was investigated experimentally. A facility was constructed to establish countercurrent shear layers without the formation of global stagnation in the flow. Particle image velocimetry was employed to obtain detailed measurements within the region of self-preservation for velocity ratios U2/U1 between 0 and -0.3. The spatial growth rate of countercurrent shear layers was found to agree generally with simple analytical theory. At 30% counterflow, the growth rate was approximately twice as large as the case with no counterflow. Peak turbulence quantities, when normalized by the applied shear magnitude, ΔU, were found to be nominally constant for low levels of counterflow, but at counterflow velocities above 13% of the primary stream velocity, peak turbulence levels increased. The observed transition is accompanied by the development of mean flow three-dimensionality. The deviation occurs at a counterflow level that is in agreement with theoretical predictions for transition from convective to absolute instability.