Nanoparticles carrying active drug substances have been used since the 70's and have undergone numerous improvements since then. Nowadays, the latest generation of nanoparticles, called lipid nanoparticles (LNPs), is used for different applications such as vaccines and cancer treatments and offer a versatile approach to delivering genetic materials like RNA. LNPs are non-viral delivery vehicles obtained by the self-assembly of lipids during the rapid mixing of an aqueous phase containing mRNA with an organic phase containing lipids. During this process, mRNA is encapsulated within the LNP due to electrostatic interaction with an ionizable lipid. Different methods to produce LNPs are described in the literature and, as of now, continuous methods are mostly used to produce LNP-encapsulated mRNA (LNP-mRNA). T-shaped mixers are commonly used to produce mRNA-LNPs. This technology can operate at two different scales: microfluidic chips which can range from tens to hundreds of microns in size, and millimetric tubing for production scale up. This review intends to describe LNP-mRNA characteristics and their production modes with a special focus on the challenges related to the mixing quality, especially during scale-up.