Methionine (Met) is a key amino acid that cannot be synthesized and must be provided in the diet of aquaculture species. It is a precursor of various metabolites and is involved in several biochemical and physiological systems, including protein synthesis, antioxidant defense, immune response, bile salt synthesis, etc. Today’s commercial aqua feeds are predominantly based on plant protein and so Met is often the first limiting amino acid. Met deficiency can be avoided by adding Met rich ingredients such as fish meal, but this approach increases the overall protein content, feed cost and finally increases the amount of ammonia excreted into the water. A more economical, healthy, and environmentally friendly option is to supplement the feed with the right amount and source of Met. Met supplements are commercially available in four forms: DL-Met, L-Met, DL-MHA (methionine hydroxyl analogue) and DL-Methionyl DL-Methionine (Met-Met dipeptide of DL-Met). DL-MHA is not an amino acid and therefore animal needs to convert to Met before being able to use them for protein synthesis. There have been studies on the bioefficacy of Met sources in various fish species. Overall, studies show that DL-MHA is not as efficient as other Met sources on an equimolar basis while DL-Met and L-Met performed equally well. Recent studies in Nile tilapia, rainbow trout and common carp provided physiological evidences in terms of transporter, absorption, and bioconversion elucidating why D-MHA is not as efficient as other Met sources. On the other hand, Met-Met showed significantly better performance relative to DL-Met in shrimp because of its low leaching and better availability. This was further supported by physiological data from recent studies. In summary, using the right Met value determined in these studies is a critical step when selecting a Met source in feed formulations.