In the context of the depletion of fossil resources, formaldehyde is an emerging C-1 source exhibiting high and versatile reactivity, in comparison to the most studied C-1 molecules (CO2, CO, HCOOH, and CH4). In the present mini-review, we show that biocatalysis is an ideal approach to control the reactivity of formaldehyde and to use its great potential as a platform for the synthesis of value-added chiral products. The ability of aldolases and ThDP-dependent enzymes to catalyze stereocontrolled carbon-carbon bond formation with formaldehyde are shown to involve aldol and umpolung reactivity. The synergetic combination of (i) enzyme discovery, (ii) mechanistic understanding, and (iii) enzyme engineering provides highly stereospecific biocatalysts of increasing interest for synthetic chemistry.