Lignin is the second most abundant biopolymer and the most abundant aromatic polymer on earth. Its effective and efficient utilization is crucial in achieving sustainable development. The recalcitrant nature of the lignocellulosic complex is the major challenge in lignin valorization. Isolation of lignin without altering its native structure is almost inconceivable. Mild-acidolysis lignin and its various modifications have been developed over the years to minimize the extensive degradation of the native lignin during its isolation. This chapter discusses the various approaches to lignin isolation from biomass, their advantages, and possible limitations. A detailed deliberation on the different mild-acidolysis processes in lignin isolation has also been included.