This chapter replies to the question- How scientists conclude if a disease is genetically determined or not? Various methods of genetic analysis like segregation analysis, twin study, and linkage analysis are described in detail. In linkage analysis determination of recombinant frequency and LOD score methods are described. Importance of various genetic markers like restriction fragment length polymorphism (RFLPs), microsatellites and single nucleotide polymorphism (SNPs) are also described in this chapter. This chapter also deals with the Genome wide association studies and method to carry out the genome wide association studies.