is the commercial successor of Bobby II by Hans-Joachim Kraas and Günther Schrüfer. It was market by ChessBase as 16-bit engine in 1995 as analysis engine for ChessBase/Windows 1.0, and in 1998 as 32-bit add-on engine of the Fritz 5.32 package  . Doctor? is written in C and was therefor available for various hardware platforms and operating systems. According to Ingo Althöfer, who used Doctor? in his triple-brain experiments, it was the first engine which implemented a k-best mode. Doctor?'s strengths were mainly due to strategy and unsual pawn structures in endgames. The engine uses fractional plies with a depth increment in iterative deepening of 1/3 ply .
- STC-Search (solution tree cost oriented search) 
- Ample chess knowledge in evaluation function and search heuristics.
- Positional playing style with tactical surprises.
- Written in C
- Implemented on Atari ST, PC (Unix, Win3.x, Win95/98, WinNT), Mac, Sun, BiiN
- Doctor! 3.0, # nodes searched, and knowledge versus speed by Roger, rgcc, January 29, 1999
- Doctor 3.0? by Swaminathan Natarajan, CCC, April 01, 2009
- Doctor? 2.0 / Engine MacIntosh - Schachprogramm - Software - Schachversand Niggemann
- Doctor 3.0 in CCRL 40/4
- Doctor Who logo used from 1963 to 1967, Cultural icon from Wikipedia
- Fritz 5.32 - mehr als nur ein Update! by Peter Schreiner (German), Schachclub Leinzell
- Doctor? 2.0 / Engine MacIntosh from Schachversand Niggemann
- Re: Doctor module for Fritz/Chessbase? by Ingo Althöfer, rgcc, December 08, 1997
- Doctor? by Dr. Hans-Joachim Kraas and Dr. Gunther Schrüfer (as of June 2018, dead link)
- Günther Schrüfer (1988). Minimax-Suchen : Kosten, Qualität und Algorithmen. Technical University of Braunschweig (German)