  We propose a bottom-up approach to the building of particle physics models
from string theory. Our building blocks are Type II D-branes which we combine
appropriately to reproduce desirable features of a particle theory model: 1)
Chirality ; 2) Standard Model group ; 3) N=1 or N=0 supersymmetry ; 4) Three
quark-lepton generations. We start such a program by studying configurations of
D=10, Type IIB D3-branes located at singularities. We study in detail the case
of Z_N, N=1,0 orbifold singularities leading to the SM group or some left-right
symmetricextension. In general, tadpole cancellation conditions require the
presence of additional branes, e.g. D7-branes. For the N=1 supersymmetric case
the unique twist leading to three quark-lepton generations is Z_3, predicting
$\sin^2\theta_W=3/14=0.21$. The models obtained are the simplest semirealistic
string models ever built. In the non-supersymmetric case there is a
three-generation model for each Z_N, N>4, but the Weinberg angle is in general
too small. One can obtain a large class of D=4 compact models by considering
the above structure embedded into a Calabi Yau compactification. We explicitly
construct examples of such compact models using Z_3 toroidal orbifolds and
orientifolds, and discuss their properties. In these examples, global
cancellation of RR charge may be achieved by adding anti-branes stuck at the
fixed points, leading to models with hidden sector gravity-induced
supersymmetry breaking. More general frameworks, like F-theory
compactifications, allow completely $\NN=1$ supersymmetric embeddings of our
local structures, as we show in an explicit example.

High Energy Physics - Theory;High Energy Physics - Phenomenology