Quantum Monte Carlo methods provide powerful numerical tools to solve exactly the many-body problem for a system of Bose particles. In particular, at zero temperature, projective Monte-Carlo methods can solve the Schroedinger equation for the many-body ground state, irrespective of dimensionality, applied external fields or confining geometries and the strength of interaction couplings. I will review the essential aspects of such numerical methods and I will discuss their application to some relevant problems in the field of ultracold atoms where interaction or quantum fluctuation effects are difficult to be treated using mean-field or perturbative approaches. These applications include: the equation of state of an interacting Bose gas, dipolar systems in two spatial dimensions and the one-dimensional two-component Bose gas with contact interactions. Some specific configurations of the latter system can be described in terms of an exactly solvable model (Lieb-Liniger model) and a direct comparison shows that quantum Monte-Carlo simulations reproduce exact results and provide extensions to a broader class of problems.