The most basic parameters describing a jet are its mass per unit length (m), mass flux (J), momentum flux (or thrust, T) and energy flux (or power, K). These parameters may be expressed in terms of the density (), velocity (v), internal energy, total stress tensor, heat flux density, gravitational potential and area (S) of a surface perpendicular to the jet (e.g., Leahy 1991). In most jet models, the momentum and energy fluxes are dominated by the kinetic motion of the jet material, in which case m = dS, J = vdS, T = v2 dS, and K = v3 / 2dS. Thus the problem of finding these parameters comes down to determining and v. It is unlikely that theorists will be able to construct realistic models of jets until and v have been directly measured by observers, ideally as close as possible to the central engine and also as a function of distance along the jet (to study, for example, entrainment effects). Extragalactic jets are observed primarily as radio synchrotron sources but such observations provide little direct information on the density and velocity. A number of indirect arguments, however, favor light, fast jets. (i) Because of the steep dependence of the power on v, luminous radio sources are most easily accommodated with fast, probably relativistic, jet flows; the mass loss rate and total mass loss from the nucleus over the radio source's lifetime are then minimized and there is no conflict with the (mostly) upper limits on from Faraday depolarization arguments. (ii) The existence of "hot spots" and "cocoons" of old jet material implies that the rate of advance of the head of the jet is substantially less than v, so < e, where e is the external density (Scheuer 1974). (iii) In a qualitative way, it seems that nonthermal processes are likely to be enhanced over thermal ones when speeds are high and densities low. (iv) If the jet is surrounded by a "cocoon" of backflowing jet material (Norman et al. 1983), surface instabilities should be less destructive to the jet than if it were surrounded by interstellar or intergalactic gas, for the "cocoon" is of low density like the jet. (v) Attempts to estimate v by dividing the (model dependent) jet power by the (model dependent) jet thrust tend to give high speeds for powerful sources like Cygnus A (e.g., Leahy 1991). (vi) Models of "bent" beams in narrow angle tail sources typically provide upper limits to the particle density in the jet of 10-6 - 10-4 cm-3 (O'Dea 1985). On the other hand, the morphologies of "bent" jets have also been successfully modeled as ballistic motion disturbed by nuclear precession or acceleration of the active galaxy by the gravity of a companion galaxy; such models (e.g., Blandford & Icke 1978) give high gas densities and relatively low jet speeds, of order hundreds to thousands km s-1. In SS433, the jet velocity of 0.26c can be measured directly from the hydrogen lines, which originate in dense gas. Lastly, the difference between Fanaroff-Riley (FR) classes I and II radio morphologies has challenged the theorist (e.g., Norman et al. 1984; Bicknell 1986) and is often accounted for in terms of higher velocities in the FRIIs (e.g., Williams 1991); recent progress in this area is reviewed by Laing in these proceedings.
About a decade ago, some astronomers hoped that studies of optical emission lines would provide definitive values for and v. Unfortunately, this expectation has not been met. In general, the jet material itself appears to be of too low density and/or too hot to emit significant optical line emission, and the emission lines that are seen originate in interstellar or intergalactic gas close to the jet. Therefore, emission-line studies provide no direct information about the jet material. Nevertheless, observations clearly indicate that interstellar and intergalactic gas may be compressed, ionized or excited by a jet. I shall separate the emission-line observations into two groups, namely those that may be interpreted in terms of radiative bow shocks driven by the head of the jet into the surrounding gas (Section 2) and those that are suggestive of interface and entrainment effects along the jet (Section 3). In practice, the distinction between these two categories is not always clear cut (e.g., jets may drive shocks into interstellar clouds encountered along their path, not just at the current jet terminus). I shall then go on to discuss in more depth the ionizing source for this gas (Section 4). In Section 5, evidence for wide angle gaseous outflows in active galaxies will be described; perhaps these outflows are analogous to the wide-angle outflows sometimes seen from pre-main sequence stars. Section 6 summarizes a recent emission-line study of the intertwined, helical jets in the spiral galaxy NGC 4258. Concluding remarks and a brief discussion of future prospects for emission-line studies of extragalactic jets are given in Section 7.