1. Introduction2. Simple supercuspidal representations of classical groups3. Characters of simple supercuspidal representations4. Arthur's local classification theorem5. Simple supercuspidal $L$-packet of $Sp_{2n}$6. Transfer factors at affine generic elements7. Endoscopic lifting of simple supercuspidal $L$-packets of $SO_{2n}^{\mu }$8. Simple supercuspidal $L$-packet of $SO_{2n+2}$ and $SO_{2n+2}^{\mathrm {ur}}$9. Application: Formal degree conjecture for simple supercuspidal $L$-packets for some bad primesA. Kloosterman sums and Gauss sumsB. Depth of simple supercuspidal representationsC. Spinor twists for $L$-packets of even orthogonal groups