AbstractmRNA vaccines have recently generated significant interest due to their success during the COVID‐19 pandemic. Their success is due to advances in mRNA design and encapsulation into ionizable lipid nanoparticles (iLNPs). This has highlighted the potential for the use of mRNA‐iLNPs in other settings such as cancer, gene therapy, or vaccines for different infectious diseases. Here, we describe the production of mRNA‐iLNPs using commercially available reagents that are suitable for use as vaccines and therapeutics. This article contains detailed protocols for the synthesis of mRNA by in vitro transcription with enzymatic capping and tailing and the encapsulation of the mRNA into iLNPs using the ionizable lipid DLin‐MC3‐DMA. DLin‐MC3‐DMA is often used as a benchmark for new formulations and provides an efficient delivery vehicle for screening mRNA design. The protocol also describes how the formulation can be adapted to other lipids. Finally, a stepwise methodology is presented for the characterization and quality control of mRNA‐iLNPs, including measuring mRNA concentration and encapsulation efficiency, particle size, and zeta potential. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC.Basic Protocol 1: Synthesis of mRNA by in vitro transcription and enzymatic capping and tailingBasic Protocol 2: Encapsulation of mRNA into ionizable lipid nanoparticlesAlternate Protocol: Small‐scale encapsulation of mRNA using preformed vesiclesBasic Protocol 3: Characterization and quality control of mRNA ionizable lipid nanoparticles