Phytochemicals are bioactive compounds derived from various plant sources, including vegetables, seeds, fruits, and grains, proven to offer benefits for safeguarding human health. The adoption of an unhealthy lifestyle significantly heightens the risk of developing lifestyle-associated diseases. This increased susceptibility may result from oxidative stress brought on by exposure to environmental pollutants, including contaminated air, water, and food, which is exacerbated by a sedentary lifestyle, smoking, drinking alcohol, and poor eating habits. Conversely, phytochemicals, encompassing diverse categories such as polyphenols, terpenes, and alkaloids, exhibit potent antioxidant properties. These compounds have the capacity to neutralize reactive oxygen species (ROS) and possess anti-inflammatory properties, thereby mitigating inflammation—the underlying factor in the development of lifestyle diseases like cardiovascular diseases (CVDs), obesity, cancer, type 2 diabetes, and neurodegeneration. Notably, several phytochemicals have been identified for their role in reducing triglyceride levels and modulating HDL/LDL ratios in CVDs, attenuating cytokine-mediated injury to beta-pancreatic cells in type 2 diabetes, preventing nuclear factor kappa B (NF-κB) from being overactivated and other inflammatory signaling cascades in cancer, and diminishing amyloid beta (Aβ) aggregation and neurofibrillary tangles (NFTs) formation while improving cognitive function in Alzheimer’s disease (AD). This chapter is dedicated to exploring the extensive research on phytochemicals and their implications in addressing lifestyle-associated human diseases. Additionally, it sheds light on the existing limitations in understanding these bioactive compounds and their application in promoting human health.