This paper presents the isolation and characterization of few-layer black phosphorus (BP) flakes with thickness down to two single layers. A modified mechanical exfoliation method was developed to increase the yield of atomically thin BP flakes. The number of layers in BP flakes was determined using optical microscopy, Raman spectroscopy, and transmission electron microscopy. The exfoliated BP flakes were found to be highly crystalline and stable even in free-standing form. Density functional theory (DFT) calculations showed a strong thickness dependence of the band structure, and the exciton binding energy was calculated for different numbers of layers. The optical gap was consistent with preliminary photoluminescence results on thin BP flakes. The environmental stability of BP flakes was studied, revealing that they are hydrophilic and that long-term exposure to air moisture etches them away. However, the aging of BP flakes is slow enough to allow the fabrication of field-effect transistors with strong ambipolar behavior. DFT calculations also provided insights into the water-induced changes in the structural and electronic properties of BP. The paper also describes the fabrication of BP field-effect transistors, showing ambipolar transport with high mobility and large on/off ratios. The study highlights the potential of BP as a promising 2D material for electronic and sensing applications due to its unique electronic and hydrophilic properties.This paper presents the isolation and characterization of few-layer black phosphorus (BP) flakes with thickness down to two single layers. A modified mechanical exfoliation method was developed to increase the yield of atomically thin BP flakes. The number of layers in BP flakes was determined using optical microscopy, Raman spectroscopy, and transmission electron microscopy. The exfoliated BP flakes were found to be highly crystalline and stable even in free-standing form. Density functional theory (DFT) calculations showed a strong thickness dependence of the band structure, and the exciton binding energy was calculated for different numbers of layers. The optical gap was consistent with preliminary photoluminescence results on thin BP flakes. The environmental stability of BP flakes was studied, revealing that they are hydrophilic and that long-term exposure to air moisture etches them away. However, the aging of BP flakes is slow enough to allow the fabrication of field-effect transistors with strong ambipolar behavior. DFT calculations also provided insights into the water-induced changes in the structural and electronic properties of BP. The paper also describes the fabrication of BP field-effect transistors, showing ambipolar transport with high mobility and large on/off ratios. The study highlights the potential of BP as a promising 2D material for electronic and sensing applications due to its unique electronic and hydrophilic properties.