Unfortunately old saved files with particles cannot be 100% correctly converted to the new system, which means you need to tweak manually quite some settings.

Here are some tips to survive:

- it needs particle cache by default to work, run first an entire cycle with it (ALT+A)
- set start/end for cache to an appropriate value
- if the cache was not yet created, starting anywhere but the first frame or going forward by more than 1 frame (up arrow) will cause the particles system to stop doing calculations. Restart from the last calculated frame to resume simulation.

Blender's particle system has been fully rewritten by Janne Karhu, adding dozens of new and improved features. This includes:

• Emitter, Hair and Reactor particle types.
• Newtonian, Keyed and Boids physics.
• Various particle visualization and rendering types.
• Hair editing with combing, growing, cutting, .. .
• Interpolated child particles from parents.
• Explode modifier.
• Harmonic, Magnetic fields, and multiple falloff types.
• Vertex group and texture control for various properties.

Particle System and Types: an overview of how particle system
s work and explanation for the emitter, reactor and hair particle types.

Various physics systems are available for the simulation of particles movements, along with extensive description of their parameters:

• None physics: this system is as much useful as its name indicates the contrary: it doesn't particles movements, so belongs to no Physics systems but is interesting for growing objects, much like old Blender's static particles
• Newtonian physics: this system is the closest to old Blender's particles system, refreshed and with new controls
• Boids physics: particles with limited artificial intelligence, including behaviour and rules programming, ideal for flocks of birds or schools of fishes, or predators vs preys simulations
• Keyed physics: no documentation for the moment. This particles systems' code will be refreshed shortly.

Visualization: particles can be visualized and rendered in different ways like points, lines, objects or billboards.

Extras: settings for effectors, control over time, vertex groups, .. .

Child Particles: child particles make it possible to create more particles from the existing particles. For example for Hair you can edit a small number of parent particles, and then generate many more hairs for the final render.

Explode Modifier: uses a particle system to explode a mesh into pieces.

Hair Editing and Softbody: hair particle systems can be edited in particle mode, and have softbody physics applied to them.

Documentation for the whole particle system does not exist yet, some info is here, and is currently being rewritten:

http://wiki.blender.org/index.php/BlenderDev/Particles_Rewrite_Doc
http://wiki.blender.org/index.php/BlenderDev/Particles_Rewrite

The new particle system cannot be backwards compatible. Old particle systems are automatically converted to the new system, but will require tweaking to get them looking the same as before.