For the past two decades there has been interest in the development of Ti-based intermetallic alloys for aircraft structural and engine applications, culminating in intense activity over the past five years. The primary driver for this interest is the increase of the strength/weight ratio for aircraft materials, and the low density, high modulus, and high melting temperature of some Ti alloys make them promising candidates. Most of the work has centered on the Ti-Al system, particularly Ti3Al (ordered DO19 structure, space group P63/mmc), TiAl (ordered Ll0, space group P4/mmm), and, more recently, an new ordered orthorhombic alloy (O-phase) based on Ti2AlNb (space group Cmcm). The primary limitation of these alloys is low ductility at room temperature, although improvements in ductility, strength, and creep resistance at low and elevated temperatures are also continuing goals. Transmission electron microscopy has played a key role in the understanding of the behavior of these alloys and has defined possible pathways for property improvement. Microscopy studies have focused on many issues, including: deformation mechanisms at low and elevated temperatures, and their relation to ductility, strength, creep, and fracture; the role of interstital elements; the role of ternary and quaternary additions; phase equilibria and stability; and the microstructure of alloys containing dispersoids or reinforcing fibers. Some highlights of these studies will be discussed briefly below.