Complex diseases are disorders that result from the combined influence of multiple genes and environmental factors rather than a mutation in a single gene. They are also known as multifactorial diseases because their development depends on complex interactions between genetic susceptibility and external influences. Unlike Mendelian disorders, which follow clear inheritance patterns, complex diseases do not exhibit predictable modes of inheritance. Instead, several genes of small effect contribute to disease risk, and environmental exposures determine whether genetically predisposed individuals actually develop the condition. Examples include diabetes mellitus, hypertension, coronary artery disease, obesity, asthma, Alzheimer's disease, osteoporosis, rheumatoid arthritis, schizophrenia, and many forms of cancer. Human population genetics plays a significant role in understanding complex diseases by examining the distribution of genetic variants within populations and identifying how genetic diversity influences disease susceptibility. Genome-wide association studies (GWAS) have identified numerous genetic loci associated with common diseases, but these loci individually contribute only a small proportion of disease risk. Therefore, environmental factors become equally important in determining disease occurrence and progression. Environmental factors include all external conditions and lifestyle influences that interact with the genome throughout life. These factors may act before birth, during childhood, or throughout adulthood. Their effects may alter gene expression through epigenetic mechanisms such as DNA methylation, histone modification, and regulation by non-coding RNAs, thereby influencing disease susceptibility without changing the DNA sequence itself. Diet is one of the most significant environmental factors.