This chapter is concerned with the ability to extract χ versus k from ln(I₀/I) (transmission) or from I_f/I₀ (fluorescence) versus E. Standard theory and analytical practice for X-ray absorption fine structure (XAFS) or extended X-ray absorption fine structure (EXAFS) derives χ from a wide range of experimental data and then presents the theoretical computation of χ(k) for nanostructural analysis. This remains one of the largest continuing challenges in theory and experiment for XAFS analysis. Empirical approaches have been used for many decades. Theoretical advances in understanding and measurement now permit greater investigation and insight, yet progress has perhaps been slow due to insufficient diagnostics on standard beamlines. This chapter will discuss advances, concerns and opportunities.