Probiotics are live microorganisms (small single-celled bacteria) that are similar to the beneficial microorganisms found in the human gut. According to the currently adopted definition by FAO / WHO, probiotics are: ‘Live microorganisms which when administered in adequate amounts confer a health benefit on the host’. Popularly known as friendly or good bacteria, these bacteria are said to be beneficial for the body. Theoretically, bacteria are categorised by genus, species and strain names. Most probiotic foods contain bacteria of the species like Lactobacillius and Bifidobacterium. These are said to benefit the host by improving the balance of the intestinal microflora, upon ingestion. They also secrete enzymes that aid in digestion and enhance the workings of immune-system cells in the digestive tract. It is said that consuming probiotics helps maintain a healthy balance of good bacteria in the digestive tract. Probiotics can have immunomodulatory and anticarcinogenic effects as well. Often, nutritionists recommend them after a course of antibiotics or as part of the treatment for gut related candidiasis.

Human body contains miniature ecology of microbes, collectively known as the gut flora. Most of these bacteria are harmless and are said to contribute to our growth and health. It is important that the balance of microbes is maintained to promote the beneficial bacteria over the potentially harmful ones. Due to the use of antibiotics, excess alcohol, stress, or exposure to toxic substances, a number of these bacterial types can get thrown out of balance. In such cases, the bacteria that suit our body can decrease, allowing the harmful ones to thrive and pose a threat to our health. This can be prevented by improving the quality of our food intake. By including foods rich in probiotics, we can maintain a healthy gut flora and thereby, avoid potential health problems.

Probiotics have a lot of benefits for our health. At times, the balance between the good and the bad bacteria can get haywire due to the use of antibiotics and other toxic substances. Foods rich in probiotics can offset their side effects and also prevent further infections. They can also ease symptoms of lactose intolerance, where certain active strains may help lactose intolerant individuals tolerate more lactose. Probiotics could also halt the unfriendly bacteria and suppress their growth and activity in conditions like diarrhoea, irritable bowel syndrome, inflammatory bowel disease, skin and vaginal infections. It is also said that certain strains of probiotics can exert anti-carcinogenic effects by decreasing the activity of enzymes that generate carcinogens. They can exert a positive influence on the gastrointestinal environment and also reduce the risk of cancer. Besides, some types of probiotics can also lower cholesterol levels, by breaking down bile in the gut and thus, inhibiting its reabsorption. The benefits of probiotics can also include their role in improving the body’s immune function. There are cells in the digestive tract that are connected to the immune system. By introducing probiotic bacteria, one can alter these microorganisms that influence the immune system's defenses and thereby help in building immunity.

There is an ongoing debate on the safety of the use of probiotics in young children, elderly people and people with compromised immune systems. Side effects of probiotics tend to be mild and digestive (like gas or bloating). But they can also be serious leading to infections and unhealthy metabolic activities, if taken in excess. Some people also question the idea that the bacteria taken by mouth can survive the process of human digestion. Thus, along with their benefits, there also exist questions that challenge some of the claims made for probiotics.

Probiotics are available in foods as well as dietary supplements. They can be found in fermented food products such as yoghurt, fermented and unfermented milk, cabbage kimchee and soybean-based miso and natto (fermented traditional Japanese health foods). Some probiotic foods date back to ancient times, such as fermented foods and cultured milk products.