The risk and prevalence of many health problems have increased as a result of changed lifestyles, demanding work schedules, and an increase in poor and unbalanced eating habits. Therefore, it is expected that natural and nutrient-rich supplements will be consumed because of which there is a greater demand for phytochemicals. Phytochemicals are naturally occurring bioactive compounds with therapeutic applications that are present in plants. The most common types of phytochemicals are terpenoids, polyphenols, phenolic components, alkaloids, carotenoids, phytosterols, saponins, and fibers which can be found in grains, seeds, nuts, legumes, and fruits and vegetables. Despite the fact that many secondary metabolites have been chemically discovered, the majority of these substances’ biological activities are still unknown. Numerous different phytochemicals that are frequently included in a person’s diet have an impact on health and could assist in preventing disease. In addition, they are important for business and industry since they give flowers, fruits, and vegetables their colors and fragrances. They are also essential components of medications and nutraceuticals. This chapter discusses the diverse classes of phytochemicals and their importance. Here we also discuss the variation in phytochemical composition with respect to agronomic inputs in the field, geographic variation and genotypic variation along with the bioaccessibility and bioavailability of phytochemicals.