  Finding life on exoplanets from telescopic observations is an ultimate goal
of exoplanet science. Life produces gases and other substances, such as
pigments, which can have distinct spectral or photometric signatures. Whether
or not life is found with future data must be expressed with probabilities,
requiring a framework of biosignature assessment. We present a framework in
which we advocate using biogeochemical "Exo-Earth System" models to simulate
potential biosignatures in spectra or photometry. Given actual observations,
simulations are used to find the Bayesian likelihoods of those data occurring
for scenarios with and without life. The latter includes "false positives"
where abiotic sources mimic biosignatures. Prior knowledge of factors
influencing planetary inhabitation, including previous observations, is
combined with the likelihoods to give the Bayesian posterior probability of
life existing on a given exoplanet. Four components of observation and analysis
are necessary. 1) Characterization of stellar (e.g., age and spectrum) and
exoplanetary system properties, including "external" exoplanet parameters
(e.g., mass and radius) to determine an exoplanet's suitability for life. 2)
Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate
habitability. 3) Assessment of potential biosignatures within the environmental
context (components 1-2) and any corroborating evidence. 4) Exclusion of false
positives. The resulting posterior Bayesian probabilities of life's existence
map to five confidence levels, ranging from "very likely" (90-100%) to "very
unlikely" ($\le$10%) inhabited.
