The eigenvalues of the combinatorial Laplacian of graphs with boundaries and infinite graphs without boundary are studied. For a graph with boundary G =( V ∪∂ V , E ∪∂ E ), a sharp lower bound of the first eigenvalue λ1( G ) is given provided G satisfies a general condition, the so called non-separation property. For an infinite graph G without boundary, the bottom of the spectrum, i.e., the infimum of the spectrum of the combinatorial Laplacian of G , denoted λ0( G ), is estimated as λ0( G ) ≤ ¼ μ( G )2exp(μ( G )),
where μ( G ) is the exponential growth of G . As a corollary, if G is subexponential, λ0( G )=0. On the contrary,λ0( G ) >0 is shown for a simply connected infinite graph G with degree ≥4 at each vertex.