How did the HDF come to be and why has it been so successful? Bob Williams convened an advisory committee comprising a number of extragalactic scientists who met here on 31st March 1995. Our brief was to be visionary and to consider the optimum science that would emerge from a significant allocation of Director's Discretionary Time. Those present will remember a rather rambling discussion which focused ultimately on one or two long exposures, both as a scientific mission and a public legacy for HST. My own notes from that meeting indicate much disagreement on details: the number of fields (north and south or just one?), the number of filters (surprisingly only one or two... nobody argued for more so far as I can recall), where to point (interesting area with a distant QSO or cluster, or a blank field?). Some of us questioned whether the community should at least be allowed to demonstrate whether it had a smarter idea than those of the gathered 'experts'. The latter proposition, unsurprisingly, did not achieve much support! What I am trying to say, in a way that does not insult fellow committee members, is that we hardly prescribed HDF at that meeting. The credit lies with Bob Williams and his team at ST ScI who turned a very sketchy idea into a carefully-planned series of observations.
Contrary to what some might imagine, the reason HDF has been so successful is not solely the depth of the image; images almost as deep have been obtained from the ground (cf. Metcalfe et al. 1995, Smail et al. 1995). The major step forward was the combination of the image quality only HST can deliver and, foremost, the multi-passband nature of the data. The use of 4 strategic filters, whose relative exposure times were carefully balanced, has been particularly successful. The UV and blue exposure times were prohibitively long for most guest observers and surprisingly little multicolour data had been obtained with HST prior to the HDF. Add to that the public interest in the beautiful colour images, free access to the data, the galvanizing effect of this large investment on astronomy's premier facility on other telescopes and you have the ingredients of the success of the HDF.