  We present a study of dense molecular gas kinematics in seventeen nearby
protostellar systems using single-dish and interferometric molecular line
observations. The non-axisymmetric envelopes around a sample of Class 0/I
protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and
PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to
construct line-center velocity and linewidth maps for all sources to examine
the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000
AU. The direction of the large-scale velocity gradients from single-dish
mapping is within 45 degrees of normal to the outflow axis in more than half
the sample. Furthermore, the velocity gradients are often quite substantial,
the average being ~2.3 km\s\pc. The interferometric data often reveal
small-scale velocity structure, departing from the more gradual large-scale
velocity gradients. In some cases, this likely indicates accelerating infall
and/or rotational spin-up in the inner envelope; the median velocity gradient
from the interferometric data is ~10.7 km/s/pc. In two systems, we detect
high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\
emission. This enables us to study the infall and rotation close to the disk
and estimate the central object masses. The velocity fields observed on large
and small-scales are more complex than would be expected from rotation alone,
suggesting that complex envelope structure enables other dynamical processes
(i.e. infall) to affect the velocity field.
