DNA microarrays enable insights into global gene expression by capturing a snapshot of cellular expression levels at the time of sample collection. Careful RNA handling and extraction are required to preserve this information properly, ensure sample-to-sample reproducibility, and limit unwanted technical variation in experimental data. This chapter discusses important considerations for "array-friendly" sample handling and processing from biosamples such as blood, formalin-fixed, paraffin-embedded samples, and fresh or flash-frozen tissues and cells. It also provides guidelines on RNA quality assessments, which can be used to validate sample preparation and maximize recovery of relevant biological information.