This book discusses the latest investigations into the electronic structure of narrow-gap semiconductors in extreme conditions, and describes in detail magnetic field and pressure measurements using two high-quality single crystals: black phosphorus (BP) and lead telluride (PbTe). The book presents two significant findings for BP and PbTe. The first is the successful demonstration of the pressure-induced transition from semiconductor to semimetal in the electronic structure of BP using magnetoresistance measurements. The second is the quantitative estimation of how well the Dirac fermion description works for electronic properties in PbTe. The overviews on BP and PbTe from the point of view of material properties help readers quickly understand the typical electronic character of narrow-gap semiconductor materials, which has recently attracted interest in topological features in condensed matter physics. Additionally the introductory review of the principles and methodology allows readers to understand the high magnetic field and pressure experiments.