| Class | Reference | Comment | 
| BeamShape | Beam Shape | vertex and beam momentum spread | 
| CellJet | Event Analysis | jet cone clustering analysis, intended for hadron collider topologies | 
| ClusterJet | Event Analysis | jet clustering analysis, intended for e^+e^- collider topologies | 
| CoupSM | Standard Model Parameters | couplings and mixing matrices | 
| CoupSUSY | SUSY Les Houches Accord | compute and store SUSY couplings, using input from SusyLesHouches | 
| DecayChannel | Particle Data Scheme | the properties of a single decay channel of particle species | 
| Event | Event Record | the complete event record | 
| Hist | Histograms | a primitive built-in histogramming package | 
| Info | Event Information | various one-of-a-kind information on the current event | 
| LHAup | Les Houches Accord | base class for Les Houches external hard-event input | 
| Particle | Particle Properties | the properties of a particle in the event record | 
| ParticleData | Particle Data Scheme | the database of particle species properties | 
| ParticleDataEntry | Particle Data Scheme | the properties of a particle species | 
| PDF | Parton Distributions | base class for all PDFs | 
| Pythia | Program Flow | the top-level class, that drives the generation process | 
| ResonanceWidths | Semi-Internal Resonances | base class for all resonance-width treatment | 
| Rndm | RandomNumbers | a primitive built-in histogramming package | 
| RotBstMatrix | Four-Vectors | rotation and boosts of four-vectors | 
| Settings | Settings Scheme | the database that regulates the behaviour of the program | 
| SigmaProcess | Semi-Internal Processes | base class for all hard-process cross sections | 
| SlowJet | Event Analysis | simple jet clustering using the kT, anti-kT or Cambridge/Aachen algorithms. | 
| SpaceShower | Implement New Showers | spacelike, i..e. initial-state showers | 
| Sphericity | Event Analysis | sphericity analysis of events | 
| SusyLesHouches | SUSY Les Houches Accord | reading in of SLHA information | 
| Thrust | Event Analysis | thrust analysis of events | 
| TimeShower | Implement New Showers | timelike, i..e. final-state showers | 
| UserHooks | User Hooks | get access to some steps of the generation process | 
| Vec4 | Four-Vectors | four-vectors |