  We study path-connectedness and homotopy groups of sets of tensors defined by
tensor rank, border rank, multilinear rank, as well as their symmetric
counterparts for symmetric tensors. We show that over $\mathbb{C}$, the set of
rank-$r$ tensors and the set of symmetric rank-$r$ symmetric tensors are both
path-connected if $r$ is not more than the complex generic rank; these results
also extend to border rank and symmetric border rank over $\mathbb{C}$. Over
$\mathbb{R}$, the set of rank-$r$ tensors is path-connected if it has the
expected dimension but the corresponding result for symmetric rank-$r$
symmetric $d$-tensors depends on the order $d$: connected when $d$ is odd but
not when $d$ is even. Border rank and symmetric border rank over $\mathbb{R}$
have essentially the same path-connectedness properties as rank and symmetric
rank over $\mathbb{R}$. When $r$ is greater than the complex generic rank, we
are unable to discern any general pattern: For example, we show that
border-rank-three tensors in $\mathbb{R}^2 \otimes \mathbb{R}^2 \otimes
\mathbb{R}^2$ fall into four connected components. For multilinear rank, the
manifold of $d$-tensors of multilinear rank $(r_1,\dots,r_d)$ in
$\mathbb{C}^{n_1} \otimes \cdots \otimes \mathbb{C}^{n_d}$ is always
path-connected, and the same is true in $\mathbb{R}^{n_1} \otimes \cdots
\otimes \mathbb{R}^{n_d}$ unless $n_i = r_i = \prod_{j \ne i} r_j$ for some
$i\in\{1, \dots, d\}$. Beyond path-connectedness, we determine, over both
$\mathbb{R}$ and $\mathbb{C}$, the fundamental and higher homotopy groups of
the set of tensors of a fixed small rank, and, taking advantage of Bott
periodicity, those of the manifold of tensors of a fixed multilinear rank. We
also obtain analogues of these results for symmetric tensors of a fixed
symmetric rank or a fixed symmetric multilinear rank.
