Multiple-planet systems are particularly valuable laboratories for testing 
models of the formation and evolution of planetary systems. For example, 
their frequency directly informs planet formation models, while their orbital 
element distributions offer clues into the relative role of several proposed 
mechanisms of dynamical interaction and constraints on planet migration 
models. I will initially review the properties of the sample of 
well-characterized multi-planet systems known to-date, and discuss what 
constraints the presently available observations provide for theoretical 
models. I will then focus on the complex technical problem of and challenges 
inherent in correctly modeling multiple planetary signals present in a 
dataset, with particular emphasis on measurements obtained with 
state-of-the-art radial-velocity facilities (e.g., HARPS) and observatories 
for high-precision astrometry (e.g., Gaia) coming online in the near future. 
I will conclude by illustrating the potential of micro-arcsecond astrometry 
for important contributions to the astrophysics of planetary systems, in 
combination with other indirect and direct detection and characterization 
techniques.