This book analyses different aspects of molecular orbital pap studies in tunnelling single molecular devices. The topics of tunnelling crossover regime and electrical probing of gap between molecular orbitals are highlighted in this book. In the first section, the analysis of direct (low bias regime that corresponds to Simmons model) and field emission, Fowler-Nordheim (FN) tunnelling, and a crossover between these regimes in molecular nanojunction are presented. In the second section, an introduction to electrical and optical molecular gap-probed measurements is presented.