  We describe the basic properties and consequences of introducing active
stresses, with principal direction along the local director, in cholesteric
liquid crystals. The helical ground state is found to be linearly unstable to
extensile stresses, without threshold in the limit of infinite system size,
whereas contractile stresses are hydrodynamically screened by the cholesteric
elasticity to give a finite threshold. This is confirmed numerically and the
non-linear consequences of instability, in both extensile and contractile
cases, are studied. We also consider the stresses associated to defects in the
cholesteric pitch ($\lambda$ lines) and show how the geometry near to the
defect generates threshold-less flows reminiscent of those for defects in
active nematics. At large extensile activity $\lambda$ lines are spontaneously
created and can form steady state patterns sustained by constant active flows.
