The supernova SN1986j resembles the prototypical Type V supernova SN1961v in the relatively slow ∼1000km/s expansion velocity, the slow light curve, and also in the Hα dominated spectrum. The optical spectrum is similar to the spectra of some novae, and some OB stars with massive winds, being characteristic of a nebular plasma at about 1010cm−3 and 104K. What makes SN1986j exceptional is its tremendous radio luminosity, the brightest radio supernova observed to date. The radio emission indicates the presence of a massive circumstellar wind, with which the SN ejecta are now colliding. Since the cooling time of the optically emitting gas is about an hour, a heat source is required to power the light curve. Shocks moving back into the ejecta offer a natural heat source, and account quantitatively for the observed luminosity and spectral character of SN1986j. The large Hα/Hβ ratio is attributed to trapping of Ly α, which pumps the n = 2 level of hydrogen, causing a finite optical depth in Balmer lines, and converting Hβ to Pα and Hα. The ratio of the derived H(n = 2) density and column density yields a size for the Hα emitting region consistent with the thickness of a cooling shock, but less than 10−7 of the 1017cm VLBI size. An important discriminant between shock models and photoionization models of the spectrum is that shocks predict Lyman 2-photon emission. The mass of the optically emitting material in SN1986j is about 1M⊙, substantially less than the 2000 M⊙ argued in the case of SN1961v by Utrobin. However, there may be, and probably is, considerably more unobserved ejecta. This material should reveal itself as the remnant of SN1986j continues to evolve.