Transfection is a powerful non-viral technology used to deliver foreign nucleic acids into eukaryotic cells, and is the method of choice for a variety of applications including studying the functional role of particular genes and the proteins they code for. By over-expressing genes to produce protein of interest and also by knocking down specific genes, researchers are able to accurately define the role of genes and the protein they encode in various cellular processes. Therefore, this powerful technology is a very vital component of the array of scientific research tools. However, the exact mechanism of action of transfection and also the numerous factors that influence the success of DNA or RNA delivery processes are not clearly understood. Hence, this chapter attempts to explain some of the popular cationic lipid/polymer-based transfection reagents for in vitro DNA/small inhibitory RNA (siRNA) delivery, mainly focusing on the protocols and critical factors to keep in mind to ensure successful delivery of nucleic acids into eukaryotic cells using these methods.