The human gastrointestinal (GI) tract is the largest surface between the host, antigens, and environment. The collection of microorganisms colonizing the GI tract is known as the gut microbiota, and has co-evolved with humans over many years to form a symbiotic relationship (Backhed, 2005; Neish, 2009). Although the microbiota is generally believed to begin developing from birth, this is challenged by some studies where microorganisms were detected in womb tissue (Aagaard et al., 2014; Rodríguez et al., 2015). The GI tract is quickly colonised upon delivery and altered by events such as antibiotic treatment, illness and, diet. Furthermore, the route of delivery also appears to affect the composition of microbiota, with lactobacilli prevailing during the first few days as a reflection of the vaginal flora (Avershina et al., 2013; Aagaard et al., 2012). In contrast, infants delivered by caesarean-section are colonised by facultative anaerobes and delayed colonisation of the Bacteroides genus (Jakobsson et al., 2013; Salminen et al., 2004). The microbiota provides many benefits such as strengthening gut integrity, formation of essential vitamins (Ramakrishna, 2013), development of intestinal epithelium (Natividad and Verdu, 2013), energy harvest (den Besten et al., 2013), protection against pathogens (Baumler and Sperandio, 2016), and immune regulation (Gensollen et al., 2016). These properties show the importance of the relationship humans have with the microbiota.Obesity is an epidemic that is increasingly prevalent around the world, with more than 650 million adults worldwide being obese in 2016 (World Health Organization, 2017) and increasing. It is defined by having a body mass index (BMI) of ? 30kg/m2 and obesity is fundamentally caused by an energy imbalance between consumption and expenditure. Thus, to combat obesity is to either limit caloric intake or increase expenditure to regain balance. Absorption of nutrients from diet can also be studied to understand how the body extracts energy from food. In this review, the role of the gut microbiota in obesity is discussed.