Black phosphorus (BP) is a layered, thermodynamically stable phosphorus allotrope with a graphitic structure. It is a semiconductor with a band gap of approximately 0.3 eV and high carrier mobilities of about 1000 cm²/V·s at room temperature. BP exhibits anisotropic optical properties and superconducting characteristics, and its layered structure allows it to be used as an anode in Li-ion batteries. Mechanical exfoliation of BP crystals has led to the isolation of few-layer and monolayer phosphorene flakes, intensifying research on this two-dimensional nanomaterial. Exfoliated, p-type semiconducting BP flakes have high mobilities, large current on/off ratios, and anisotropic transport. BP shows promise as a nanomaterial that could complement or exceed the electronic, spintronic, and optoelectronic properties of graphene. However, questions remain regarding the nature of defects, contacts, doping, band transport, and the chemical stability of BP and phosphorene. Although graphene is chemically inert, other 2D nanomaterials such as graphane, fluorinated graphene, and silicene have lower chemical stability. The reduced chemical stability of these materials is related to the energetics needed to maintain stable bonding configurations, which is affected by electrostatics and structural buckling. Thus, ambient stability is likely to be a concern for BP since the phosphorus atoms have free lone pairs and valence bond angles of 102°. Early work involving ambient scanning tunneling microscopy (STM) measurements on bulk BP revealed the formation of pits and bubbles, presumably arising from electrochemical reactions between the STM tip meniscus and BP crystal. While this induced chemical degradation of bulk BP provides reason for concern, the reactivity of thinner, exfoliated BP flakes in ambient conditions and the effect of chemical reactions on device-relevant electronic properties are completely unknown. This study examines how exfoliated BP degrades to oxygenated phosphorus compounds in ambient environments through a comprehensive suite of microscopy and spectroscopy techniques, including atomic force microscopy (AFM), electrostatic force microscopy (EFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Additionally, the study investigates how BP field-effect transistors (FETs) degrade following ambient exposure and shows that atomic layer deposition (ALD) of AlOx overlayers is an effective, scalable strategy for passivating BP flakes and FETs from ambient deterioration. The study finds that BP degrades upon exposure to ambient conditions, forming oxidized phosphorus species. This degradation is more rapid on hydrophobic substrates such as octadecyltrichlorosilane self-assembled monolayers and H-Si(111) compared to hydrophilic SiO2. UnencapsulatedBlack phosphorus (BP) is a layered, thermodynamically stable phosphorus allotrope with a graphitic structure. It is a semiconductor with a band gap of approximately 0.3 eV and high carrier mobilities of about 1000 cm²/V·s at room temperature. BP exhibits anisotropic optical properties and superconducting characteristics, and its layered structure allows it to be used as an anode in Li-ion batteries. Mechanical exfoliation of BP crystals has led to the isolation of few-layer and monolayer phosphorene flakes, intensifying research on this two-dimensional nanomaterial. Exfoliated, p-type semiconducting BP flakes have high mobilities, large current on/off ratios, and anisotropic transport. BP shows promise as a nanomaterial that could complement or exceed the electronic, spintronic, and optoelectronic properties of graphene. However, questions remain regarding the nature of defects, contacts, doping, band transport, and the chemical stability of BP and phosphorene. Although graphene is chemically inert, other 2D nanomaterials such as graphane, fluorinated graphene, and silicene have lower chemical stability. The reduced chemical stability of these materials is related to the energetics needed to maintain stable bonding configurations, which is affected by electrostatics and structural buckling. Thus, ambient stability is likely to be a concern for BP since the phosphorus atoms have free lone pairs and valence bond angles of 102°. Early work involving ambient scanning tunneling microscopy (STM) measurements on bulk BP revealed the formation of pits and bubbles, presumably arising from electrochemical reactions between the STM tip meniscus and BP crystal. While this induced chemical degradation of bulk BP provides reason for concern, the reactivity of thinner, exfoliated BP flakes in ambient conditions and the effect of chemical reactions on device-relevant electronic properties are completely unknown. This study examines how exfoliated BP degrades to oxygenated phosphorus compounds in ambient environments through a comprehensive suite of microscopy and spectroscopy techniques, including atomic force microscopy (AFM), electrostatic force microscopy (EFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Additionally, the study investigates how BP field-effect transistors (FETs) degrade following ambient exposure and shows that atomic layer deposition (ALD) of AlOx overlayers is an effective, scalable strategy for passivating BP flakes and FETs from ambient deterioration. The study finds that BP degrades upon exposure to ambient conditions, forming oxidized phosphorus species. This degradation is more rapid on hydrophobic substrates such as octadecyltrichlorosilane self-assembled monolayers and H-Si(111) compared to hydrophilic SiO2. Unencapsulated