July 1, 1997
      
                  
        
  Author(s)
  Paul S. Julienne,   Kevin  Jones,   Paul D. Lett,   William D. Phillips,   Eite  Tiesinga,   U  Volz,   Carl J. Williams
 
       
            
    
    
        Cold collisions between trapped atoms are very important in the context of Bose-Einstein condensation. The meaning of the scattering length, and its determination through the precision measurements of photoassociation spectroscopy, are described. This is