This review describes a recently developed method for enzymatic synthesis with mainly undissolved substrates at very high concentrations. At the end of the reaction, up to 80% (w/w) reaction mixture is product, a fact which is promising in terms of industrial applications and large-scale systems. In comparison with other approaches for enzymatic peptide synthesis, this method gives a very high mass of product combined with good reaction yields and rates. We will analyze the historical development from two different directions which have been reported; these are described with terms like "solid-to-solid conversion" and "heterogenous eutectic mixtures". The reaction requires and takes place in a liquid phase which, however may be of very small volume. This review analyzes in detail the function of additional water or organic solvents (around 10% w/w). The liquid phase formed is usually dependent on these third components; however, it can also result from a eutectic two-substrate mixture, We summarize what is known so far about the thermodynamics and kinetics. Finally, the possibility for and main problems of scale up are discussed.