The digestion of samples is an important aspect of routine analysis and is often the limiting factor in obtaining accurate results, especially for geologic sample analysis. The interest in using different digestion methods has increased significantly during recent decades. Although sample digestion is a popular research theme for analytical geochemists, the development of sample digestion methods has lagged far behind developments in analytical instrumentation. In addition to general considerations for sample digestions, specific focus here is on open vessel digestions, closed vessel digestions, microwave digestions, partial dissolutions, dry ashing, alkali fusions, fire assays, Carius tube methods, high-pressure asher (HPA-S), and direct rock fusion techniques. This chapter addresses concerns and discusses the attributes and limitations of these sample digestion methods along with examples of their application to geochemical and environmental analysis. It is evident that only the appropriate sample digestion methods can provide high-quality analytical results and guarantee the success of further investigations.