Laboratory-scale bioreactors represent an important part of microbiological, biotechnological, and biochemical researches. The small volume bioreactors enable the biomass growth, extracellular metabolite production, solid-state fermentation, and biocatalytic processes in a controlled open system. Using the lab-scale bioreactors, the processes which were optimized in experiments can be scaled up to a pilot level. In turn, the manufacturing scale processes can be scaled down to study the heterogeneity of the system and its effect on the producer. This mini-review is focused on a short description of features of the most popular designs of continuous and semi-continuous bioreactors applied at the present time as well as their application area.