Background: Different molecular mechanisms of on target and off target primary and secondary resistance have been observed in EGFR mutant NSCLC patients after first(1st), second(2nd) and third (3rd) generation of tyrosine kinase inhibitors(TKIs). Next generation sequencing(NGS) offers a comprehensive method of detecting these mechanisms to decide next line of treatment. Methods: We retrospectively analyzed 430 samples of NSCLC for primary and secondary resistance to 1st, 2nd and 3rd TKIs. NGS was performed using thermofischer Ion Torrent Oncomine Focus 52 gene Assay. These cases were divided into 4 groups.1)Primary resistance to first and second generation TKIs 2)Primary resistance to 3rd generation TKI 3)Secondary resistance to 1st and 2nd generation TKI 4) Secondary resistance to 3rd generation TKI.Last group was further subgrouped into A when 3rd generation TKI was offered as second line after 1st or 2nd generation TKIs on detection of T790M and subgroup B when it was given as first line. Results: Group1 had 13 cases. There were 2 cases of complex EGFR exon 19 mutation p.Glu746_Leu747delinsValPro, 4 cases of EGFR exon 20 insertion, 1 case of dual EGFR L833V & H835L mutation, 2 cases with EGFR amplification with EGFR exon 19 del and PIK3CA C420_P421del along with EGFR exon 19 del. Four cases had no additional abnormality. Group 2 had 5 cases:1 case had L858R and E709A dual mutation, 2 cases had KRAS G13C and KRAS G12V along with EGFR exon 19 del. One case had EGFR amplification and one case had MET amplification along with EGFR exon 19 del respectively.Group 3 had 34 cases including 10 cases of EGFR L858R and 24 cases of exon 19 deletion.T790M mutation was detected in 8 patients, MET amplification in 7 cases,one case had both T790M and MET amplification. One case lost the primary EGFR exon 19 del. Others mutations detected were KRAS G13C, PIK3CA H1047R, TP53 R213Q and TP53 C242fs. Group3 had 15 cases with 7 cases in subgroup A and 9 cases in subgroup B. In subgroup A T790M mutation was lost in 6 out of 7 cases.One case which lost T790M developed ALK translocation.One case of EGFR exon 19 del retained EGFR T790M with EGFR C797S in cis allele. Other mutations detected were PIK3CA E542K and KRAS G12C. In subgroup B one case showed EGFR C797S(both cis and trans) besides the primary EGFR exon 19 del. One case showed BRAF G469A along with EGFR exon 19 del. Other mutations detected were CTNNB1 D32N, KRAS G12V, and PIK3CA E542K. Conclusions: Primary and secondary acquired resistance is unavoidable in EGFR mutant advanced NSCLC on any generation of TKIs. NGS offers an advantage in diagnosing mechanism of resistance for further choice of therapy.