Uncovering DNA sequence variations that correlate with phenotypic changes, e.g., diseases, is the aim of sequence variation studies. Common types sequence variations are Single nucleotide polymorphism (SNP, pronounced snip).SNPs are the third-generation molecular marker. SNP represents a DNA sequence variant of a single base pair with the minor allele occurring in more than 1% of a given population. There are an approximately 56,000,000 SNPs in the human genome. The broad field of applications for SNPs induced a pressing need for effective instruments for SNP detection. During the last decade a considerable number of methods for SNP discovery (search for new SNPs) and detection (recognition of already known SNPs) were developed. Studying the association between quantitative phenotype and SNPs is an important problem in biology. To understand underlying mechanisms of complex phenotypes, it is often necessary to consider joint genetic effects across multiple SNPs. In this article, SNPs and their role in association studies were reviewed.