Chance plays a big role in the microcosm. When two particles are hurled against each other with high energy, a collision can lead to long and complicated chains of random processes, which in the end can give hundreds of newly created particles moving out in different directions. No two collisions are identical, even if they may share common patterns. It is like a parlour game on a huge and complicated playing field, where the random choice of the dice is complemented by special rules for many squares. In the particle world the dice is called "quantum field theory", the rules are called "the Standard Model", and the playing field is a region approximately 0.0000000001 mm across. There are problems, however: even if the rules can be written in mathematical formulae so compact that they do not even fill a single sheet, the mathematics still is so complicated that nobody knows how to apply it. Often an approximate approach based on perturbation theory works, but for the strong force, QCD, this is usually not enough. We then need models, which we believe are close to the truth, but still are sufficiently simple to be useful, and where different assumptions can be varied in a controlled manner. Such a model begun to be developed in Lund, in the group around professors Bo Andersson and Gösta Gustafson, and is accordingly called "the Lund model". One way to study this model, which rapidly proved to be the best one, is to let a computer simulate collisions according to the imagined set of rules, and produce final states with properties that can be directly compared with experimental events on a statistical basis. Such computer programs are called Monte Carlos, after a known place where chance reigns. My Ph.D. thesis 1982 was based on the development of the first Lund Monte Carlo, from physics ideas to code. Gradually the description has been refined, and extended to encompass almost all known aspects of collision processes. Again it has meant the development of new physics models, and finding smart ways to implement these as working code. Comparisons with existing data, and suggestions for new studies with matching predictions, can help separate promising ideas from dead ends. The computer programs have come to be used by experimental particle physicists across the world, for comparisons with and interpretations of data. In the search for new physics phenomena, beyond the today accepted Standard Model, the programs can predict the consequences of different hypothetical scenarios. The interesting effects often are tiny, so lessons from computer simulation are relevant for detector design and search strategies.