Background: Milk kefir (MK) and water kefir (WK) are traditionally produced from different unique probiotic-containing semi-soft particles known as milk kefir grains and water kefir grains. There is significant confusion between these kefir grains, even in peer-reviewed scientific publications. Even though both grains have some similarities, they have significant differences in structure, microbial content, and impact on the finished beverage product.