The first three of the peculiar semi-stellar emission nebulae now known as Herbig-Haro objects were detected by Herbig (1948, 1951) and by Haro (1950, 1952). The initial discoveries were made in the region south of the Orion Nebula, but a total of about 40 such Objects have now been found in Orion, Taurus, Perseus, and elsewhere. All known examples occur in heavily-obscured regions that are also rich in T Tauri stars. A number of other very small nebulous spots have been observed by Haro (1953, Table 2), Méndez (1967) and others, particularly near the Orion Nebula, but these are spectro-scopically distinct from the H-H Objects in having either a strong continuum especially in the near infrared, or a rather conventional emission spectrum. The H-H Objects have quite characteristic emission spectra: the H emission lines are strong, and [0 I] and [S II] are unusually intense. The [N II] lines are also strong, and in those Objects not too heavily reddened, [O II] ??3726-29 as well. These properties can be explained (Böhm 1956, Osterbrock 1958) if H and O are only partially ionized, and if T? ? 7500° and n? ? ? 3 × 103 cm?3. Only the brightest of the H-H Objects, Herbig No. 1 (= Haro 11a, near NGC 1999) has been observed in any detail. It shows a number of weaker emission lines that are not ordinarily found in appreciable strength in gaseous nebulae: H and K of Ca II, the infrared [Ca II] lines, Mg I ?4571, and lines of [Fe II] and [Fe III]. It is significant that many T Tauri stars show, in integrated light, nebular lines very similar to those of the H-H Objects.