  State-of-the-art deep learning systems often require large amounts of data
and computation. For this reason, leveraging known or unknown structure of the
data is paramount. Convolutional neural networks (CNNs) are successful examples
of this principle, their defining characteristic being the shift-equivariance.
By sliding a filter over the input, when the input shifts, the response shifts
by the same amount, exploiting the structure of natural images where semantic
content is independent of absolute pixel positions. This property is essential
to the success of CNNs in audio, image and video recognition tasks. In this
thesis, we extend equivariance to other kinds of transformations, such as
rotation and scaling. We propose equivariant models for different
transformations defined by groups of symmetries. The main contributions are (i)
polar transformer networks, achieving equivariance to the group of similarities
on the plane, (ii) equivariant multi-view networks, achieving equivariance to
the group of symmetries of the icosahedron, (iii) spherical CNNs, achieving
equivariance to the continuous 3D rotation group, (iv) cross-domain image
embeddings, achieving equivariance to 3D rotations for 2D inputs, and (v)
spin-weighted spherical CNNs, generalizing the spherical CNNs and achieving
equivariance to 3D rotations for spherical vector fields. Applications include
image classification, 3D shape classification and retrieval, panoramic image
classification and segmentation, shape alignment and pose estimation. What
these models have in common is that they leverage symmetries in the data to
reduce sample and model complexity and improve generalization performance. The
advantages are more significant on (but not limited to) challenging tasks where
data is limited or input perturbations such as arbitrary rotations are present.
