  The idea that we live in a Universe undergoing a period of acceleration is a
strongly held notion in cosmology. As this can, potentially, be explained with
a modification to General Relativity we look at current cosmological data with
the purpose of testing aspects of gravity. Firstly we constrain a
phenomenological model (mDGP) motivated by a possible extra dimension. This is
characterised by $\alpha$ which interpolates between (LCDM) and (the Dvali
Gabadadze Porrati (DGP) model). In addition, we analyse general signatures of
modified gravity given by the growth parameter $\gamma$ and power spectrum
parameter $\Sigma$. We utilise Weak Lensing data (CFHTLS-wide) in combination
with Baryon Acoustic Oscillations (BAOs) and Supernovae data. We show that
current weak lensing data is not yet capable of constraining either model in
isolation. However we demonstrate that this probe is highly beneficial, for in
combination with BAOs and Supernovae we obtain $\alpha < 0.58$ and $\alpha <
0.91$ at $1\sigma$ and $2\sigma$, respectively. Without the lensing data no
constraint is possible. Both analyses disfavour the flat DGP braneworld model
($\alpha = 1$) at over $2\sigma$. We highlight these are insensitive to
potential systematics in the lensing data. For the growth signature $\gamma$ we
show that, in combination, these probes do not yet have sufficient constraining
power. Finally, we look beyond these present capabilities and demonstrate that
Euclid, a future weak lensing survey, will deeply probe the nature of gravity.
A $1\sigma$ error of 0.104 is found for $\alpha$ ($l_{max} = 500$) whereas for
the general modified signatures we forecast $1\sigma$ errors of 0.045 for
$\gamma$ and 0.25 for $\Sigma_{0}$ ($l_{max} = 500$), which is further
tightened to 0.038 for $\gamma$ and 0.069 for $\Sigma_{0}$ ($l_{max} = 10000$).
