Most of our knowledge about the structure of atoms and molecules is based on the analysis of the radiation emitted or absorbed by them. Among the many laser spectroscopic techniques that have revolutionized the spectroscopy field, multi-photon and multi-step are certainly one of the most important. Furthermore, the dye lasers have made it possible to selectively excite the even or odd parity levels. The book contains useful information about Rydberg atoms, multi-step excitation and quantum defect theory. The research work presented in this book is related to the investigation of the even parity bound and auto-ionizing spectra of tin using the two-step laser excitation technique in conjunction with high temperature atomic beam apparatus. The analysis was performed using the multi-channel quantum defect theory (MQDT). This book will be useful for the students of laser spectroscopy and related fields.