This document discusses advanced next-generation sequencing (NGS) library preparation methods for challenging samples. It introduces NGS applications using various sample types and sizes. Standard NGS library preparation kits have input requirements that cannot accommodate many sample types and quantities. New techniques optimized for sub-nanogram inputs can construct high-quality libraries from as little as 10 picograms of DNA. These methods show consistent conversion across a wide input range and enable applications like ancient DNA analysis and circulating tumor DNA detection from blood. The document focuses on cell-free DNA library preparation challenges and solutions like the QIAGEN cfDNA library kits.