This chapter discusses the concept of learning-by-doing in the technology development process and reviews some of thechallenges encountered when measuring learning effects. The review draws on lessons from the wind power industry, andreveals that these challenges will be linked to (a) the incorporation of other causes of cost reductions beyond that ofcumulative experience, e.g., R&D, scale effects, and exogenous technological change; (b) the geographical domain of learningand R&D spillovers; and (c) the character and the complexity of the technological development process, including theinteraction and feedback effects between learning-by-doing and R&D. This is followed by a brief discussion about policyimplications.