  In this review we discuss spin and charge transport properties in
graphene-based single-layer and few-layer spin-valve devices. We give an
overview of challenges and recent advances in the field of device fabrication
and discuss two of our fabrication methods in more detail which result in
distinctly different device performances. In the first class of devices, Co/MgO
electrodes are directly deposited onto graphene which results in rough
MgO-to-Co interfaces and favor the formation of conducting pinholes throughout
the MgO layer. We show that the contact resistance area product (R$_c$A) is a
benchmark for spin transport properties as it scales with the measured spin
lifetime in these devices indicating that contact-induced spin dephasing is the
bottleneck for spin transport even in devices with large R$_c$A values. In a
second class of devices, Co/MgO electrodes are first patterned onto a silicon
substrate. Subsequently, a graphene-hBN heterostructure is directly transferred
onto these prepatterned electrodes which provides improved interface
properties. This is seen by a strong enhancement of both charge and spin
transport properties yielding charge carrier mobilities exceeding 20000
cm$^2$/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss
several shortcomings in the determination of both quantities which complicates
the analysis of both extrinsic and intrinsic spin scattering mechanisms.
Furthermore, we show that contacts can be the origin of a second charge
neutrality point in gate dependent resistance measurements which is influenced
by the quantum capacitance of the underlying graphene layer.
