Necrotic enteritis (NE) is a major enteric disease in commercial poultry and manifests itself in clinical and subclinical forms. Despite years of research, NE is still among the leading diseases with the greatest economic impact on poultry production. Subclinical forms lead to poor performance (reduced feed intake, weight gain and eventually higher feed conversion ratio) but usually occurs with low mortality rates. The use of antibiotic growth promoters (AGPs) is proving to be an effective tool in maintaining gut health and modifying gut microbiota, thus improving broiler performance and reducing NE. Removal of AGPs has led to an increase in NE occurrence, particularly the subclinical forms. The lack of alternative strategies to control NE is mainly due to limited insight into the relationship between NE pathogenesis, the host microbiome and its immune responses. Therefore, key to overcoming NE is to define the cellular and molecular mechanisms that are involved in the progression of the disease, especially with regard to mucosal immune responses and gut microbiome. Also, assessing the impact of these changes on gut cell metabolism and function is of great importance. This information would be a valuable guide to prevent the onset or alleviate the negative impact of NE on bird's health and performance. In a series of experiments conducted for this project, the effect of single or multi-strain probiotics as well as multi-component additives were tested during NE challenge in order to define the cellular and molecular mechanisms that are involved in the progression of the disease. Results of these experiments revealed that challenging broilers with a 'naturally occurring' NE led to differential expression of tight junction (TJ) proteins in the jejunum compared to non-challenged birds. Supplementation of certain additives reduced NE lesion scores, improved performance and increased mRNA abundance of claudin-3, a key epithelial TJ protein. Multi-strain probiotics and multi-component additives (including a symbiotic and a product containing probiotics, prebiotics and essential oils) were more effective than single-strain probiotics or prebiotics. The aforementioned additives were also more effective in modulating immune responses to NE, especially by decreasing the mRNA abundance of IFN-γ and IL-10 in the jejunum. Furthermore, supplementation of these additives led to an increase in the expression of nutrient transporters (SGLT-1) and regulators of energy metabolism (PGC-1α, mTOR and AMPK); thus, improving nutrients absorption and metabolism. Microbial profiling using 16S rRNA sequencing showed that supplementation of each specific additive led to a signature-like microbiome in the ileal scrapings of broilers. However, supplementation of multi-component additives (including a symbiotic and a product containing probiotics, prebiotics and essential oils) modified the ileal microbiome in association with lower NE lesion scores, better performance and modulated immune responses. These additives reduced the relative abundance of bacteria such as ASF356, Bacteroides, Clostridium sensu stricto 1, Faecalibaculum, Lachnospiraceae UCG-001, Muribaculum, Oscillibacter, Parabacteroides, Rikenellaceae RC9 gut group, Ruminococcaceae UCG-014, and Ruminiclostridium 9 and increased the relative abundance of Lactobacillus compared to NE challenged birds. Collectively, these data indicate that during a subclinical naturally occurring NE, the use of multi-strain probiotics or multi-component additives, compared to single-strain probiotics or prebiotics, would be a more promising strategy in alleviating the effect of this enteric disease. / Doctor of Philosophy / Necrotic enteritis, an enteric disease, is one of the major diseases that negatively impacts the poultry industry and producers' profitability. The growing ban on the use of antibiotics that were used to prevent this disease has increased the number of necrotic enteritis outbreaks worldwide. Having a better understanding of the cellular and molecular mechanisms that are involved in the onset of this disease is of crucial importance and could lead to finding more effective ways to control this disease without drugs. The gut is the site of digestion and absorption of nutrients so any damage would lead to poor bird performance. In a series of experiments conducted for this project, several combinations of beneficial bacteria and nutrient sources that help bacterial growth in the gut (prebiotics) improved gut health leading to better performance during the grow-out period (days 0-42) when birds reach market age. These supplements protected the gut lining and reduced damages due to necrotic enteritis with less severe lesions. Barrier function of the gut was also improved by supplementing the diet with combination of beneficial bacteria and nutrients that help their growth in the gut. There are special types of proteins (called tight junctions) that seal up the space between intestinal cells (enterocytes) and prevent pathogens in the gut lumen from entering the body, thus preventing inflammation and disease. This helps the body to use the absorbed nutrients for growth rather than spending energy to fight pathogens, which collectively results in better growth performance. Concurrent supplementation of beneficial bacteria plus nutrients that help their growth balanced the immune responses in the gut by increasing the copy number of cytokines. Cytokines are proteins that orchestrate immune responses that the host mounts against pathogens. Certain cytokines regulate such responses by preventing the immune system from overreacting and mounting unnecessary reactions, thus preserving energy and nutrients for growth while reducing inflammation. Nutrient uptake from the gut lumen is facilitated by nutrient transporter proteins that reside on intestinal cells (enterocytes). Birds concurrently supplemented with beneficial bacteria and nutrients that help their growth in the gut increased the abundance of these proteins, resulting in improved nutrient uptake and performance compared to the control birds. Co-supplementation of beneficial bacteria and nutrient sources that help their growth modified the type and number of bacteria that are present in the gut lumen. The modified bacterial community were able to produce metabolites such as butyrate and propionate, which are beneficial for the health and growth of the intestinal cells, thus improving the bird's health and its performance. Overall, compared to beneficial bacteria alone, co-supplementation of beneficial bacteria with the nutrients that help their growth in the gut significantly reduced intestinal lesions and improved performance of broiler chickens during the production period. Moreover, dietary addition of these supplements improved gut barrier function by regulating the gene expression of tight junction proteins and gut mucosal immune responses as well as modifying the bacterial community of the gut. Therefore, such combination supplements hold promise in controlling necrotic enteritis in poultry and sustain good overall performance that translates into higher profitability to producers.